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Chromosomal Theory of Inheritance

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Biology

Did you know that corn used to be barely edible? In 7000 BC, corn was small and dry with tough shells that only contained a couple of kernels. If you look at pictures of the teosinte plant, you may not even recognize that this plant evolved into modern-day corn. Through thousands of years of agriculture and selective breeding, the corn that we consume today is ten times larger, it is much easier to grow and peel, and its sugar content has tripled. How has this happened? The answer is through selective breeding.

Selective breeding chooses parents with desirable traits, so those traits are more likely passed down to their offspring.

Like plants, humans pass genetic information through chromosomes from parent to child. While humans are not artificially selected like corn, the mechanisms by which traits are passed from one generation to the next are the same. This section will explore how genetic information is passed through generations via chromosomes.

Chromosome Theory of Inheritance Definition

The chromosomal theory of inheritance states that genetic material is passed from parents to offspring through chromosomes.

What is a Chromosome?

Genes code for every physical and biochemical trait a living organism possesses. Genes are segments of DNA that code for heritable traits. DNA is very long and located in the nucleus of every cell. If we stretch out all the DNA in one cell, it is estimated that it will be over 6 feet long. The cell itself is very small, so to fit 6 feet of DNA into the nucleus of one cell, we need to wrap and coil it. Once the DNA is wrapped, the structure is called a chromosome.

Importantly, humans don't wrap all their DNA into one giant chromosome. Instead, we segment our DNA into multiple smaller chromosomes. Typically, humans have 46 chromosomes in all somatic cells of the body. Each human possesses two copies of each chromosome, one from our mother and one from our father, except for male sex chromosomes (in which a male has one X and one Y chromosome). These two copies are called homologous chromosomes. Therefore, it can be said that a human carries 23 pairs of homologous chromosomes and two copies of every gene.

Genes are located on chromosomes, and each person has a pair of homologous chromosomes (one from each parent). Therefore, each person has two copies of each gene.

How do Chromosomes Determine Traits?

While homologous chromosomes carry the same gene, each chromosome may have alternative forms of that gene called alleles. For example, one allele of the dimples gene may code for cheek dimples, while another allele may code for no cheek dimples. Whether or not this child has dimples, the observable trait is called a phenotype, while the genetic combination of alleles for a gene is called a genotype.

A genotype is the combination of alleles for one gene. The phenotype is the physical manifestation of the genotype.

Since the chromosomes are responsible for carrying the genetic information, the Chromosomal Theory of Inheritance definition is that genetic material is passed from parents to offspring through chromosomes.

Boveri and Sutton's Chromosome Theory of Inheritance

The idea that chromosomes carry genetic information is a fairly new discovery made in the early 1900s. Although Mendel's laws of Independent Assortment and Segregation were generally accepted and the existence of chromosomes was known, it wasn't until 1902 that Theodore Boveri and Walter Sutton identified chromosomes, not proteins, as the carriers of genetic information from one generation to the next.

Before 1902, it was widely accepted that proteins carried genetic information. The reasoning was that proteins performed all the functions of the cell, therefore, the proteins would be passed on to perform the functions of the cell in the next generation. We now know that is not true, but that was a widely accepted belief. In 1902, however, Boveri observed that an embryo could not develop unless chromosomes were present. In the same year, Sutton observed the separation of chromosomes into daughter cells during meiosis. These two independent observations led to the development of the Chromosome Theory of Inheritance, which pinned chromosomes as the carriers of genetic information from parent to offspring.

Principles of the Chromosome Theory of Inheritance

The Chromosome Theory of Inheritance follows three core principles:

  1. Fertilization
  2. The Law of Segregation
  3. The Law of Independent Assortment

In this section, we will try to explain the results of Mendel's experiments according to chromosome behavior.

Fertilization

Chromosomes from each parent combine during a process known as fertilization. Sperm contains 23 chromosomes from the father, and the egg contains 23 chromosomes from the mother. During fertilization, the chromosomes give rise to the 46 chromosomes in humans. Therefore, just like Mendel's genes were made up of two alleles, chromosomes come in pairs known as homologous chromosomes, with each allele coming from one parent.

The Law of Segregation

The Law of Segregation states that only one of the two alleles is passed onto the gamete randomly. Mendel saw that when he crossed parent peas that were both Yy, he produced offspring with a 3:1 ratio of green to yellow seeds. If we recall the Punnet square in figure 1, we can see that each parent pea only passed on only one allele, either Y or y. Similarly, most human cells have 46 chromosomes, but a gamete only has 23 chromosomes following meiosis. Since each cell has pairs of each chromosome, the Law of Segregation states that only one of two chromosomes in the pair is passed onto the gamete (Figure 1).

A gamete is either a sperm or egg cell.

Figure 1: A Punnett square demonstrating the law of segregation. Source: Steve Berg

The Law of Independent Assortment

The Law of Independent Assortment states that the alleles for different genes are sorted into gametes independently from each other. For example, having an allele for dimples does not influence the sorting of alleles for eye color. Mendel's dihybrid cross experiment showed that when he crossed parent peas with an AaBb genotype, the offspring had AB, Ab, aB, and ab alleles in equal ratios suggesting that pea color did not affect the pea-size. Similarly, when genes are located on different chromosomes, each chromosome pair will be selected independently of other chromosome pairs. For example, the segregation of chromosome 1 during meiosis will not influence the segregation of chromosome 2.

Chromosomal Theory of Inheritance Summary

A summary of how Mendel's experiments support the Chromosomal Theory of Inheritance is shown below (Table 1):

Table 1: Summary of how Mendel's experiments are supported in the chromosomal theory of inheritance.

Mendel's ExperimentsChromosomal Theory of Inheritance
Fertilization Each gene is made up of two alleles Chromosomes come in pairs, with each chromosome containing one allele
Law of segregationEach parent pea with a Yy genotype either passes on a Y or y allele to its gamete at random. A human with 23 pairs of chromosomes produces a gamete containing 23 single chromosomes by randomly selecting one of the two chromosomes in each pair during meiosis.
Law of independent assortmentThe sorting of alleles for pea-size does not affect the sorting of the alleles for pea color.Each chromosomes pair will be sorted independently of other chromosome pairs during meiosis.

Ultimately, these three principles of the Chromosome Theory of Inheritance explain how genes are passed from one generation to another. Moreover, the theory helps explain the increase in genetic variation from parent to offspring. Without the three principles of the Chromosome Theory of Inheritance, we would be identical clones of our parents.

Chromosomal Theory of Inheritance Examples

While the Chromosomal Theory of Inheritance can help explain Mendelian genetics, it can also explain some unique phenomena of heredity like mutations, sex-linked genes, linked genes, and chromosomal disorders.

Mutations

Mutations are changes in the DNA sequence. Some mutations are silent, meaning that the change in sequence does not cause any phenotypic change. But other mutations are not silent, meaning that they cause an alteration in a trait. While a change in DNA sequence sounds like an impairment, it may actually be beneficial, such as the mutation causing sickle-cell anemia conferring malaria resistance. Regardless, mutations in gametes can be inherited in offspring. Recall that, during meiosis, the law of segregation states that only one of the two alleles is passed onto the gamete at random. If the chromosome passed onto the gamete contains a mutation in a specific gene, that mutation will also be passed onto the offspring. Conversely, the chromosome that does not contain the mutation can also be passed on as well.

Sick cell anemia is caused by a mutation in a single gene that changes the shape of red blood cells from circular to crescent moon-shaped. This change in shape limits red blood cells from effectively transporting oxygen throughout the body, which can be fatal. However, in countries with a high malaria prevalence, being an asymptomatic carrier for sickle-cell anemia happens to be advantageous to survival. Malaria is a mosquito-borne disease that spreads through the body via infected red blood cells. Carriers of sickle cell anemia have their red blood cells broken down much more frequently, preventing malaria from spreading rapidly.

Sex-linked Genes

As we discussed, genes are found on chromosomes, including sex chromosomes, responsible for determining our biological sex. Biological females have two X-chromosomes (XX) in humans, while biological males have one X and one Y chromosome (XY). Genes found on the Y chromosome will be inherited only in male offspring, while genes found on the X-chromosome will be inherited by both male and female offspring. Ultimately, if genes are located on the sex chromosome, it changes how genes are inherited between males and females.

Linked Genes

The Law of Independent Assortment states that alleles for different genes are sorted into gametes independently from each other. However, that is not always the case. The Law of Independent Assortment is true for genes on different pairs of chromosomes, but what if two genes are located very close together on the same chromosome?

We know that one chromosome in a chromosome pair is passed onto the gamete during meiosis. However, if the two genes are located close to each other, both of those genes (and their alleles) from the same chromosome will be inherited together. Therefore, if two genes are close together, we should expect to see two genes inherited together much more frequently than expected.

Chromosomal Disorders

Rather than mutations of single genes, there are diseases that are caused by mutations in large regions of the chromosome itself. For example, if a pair of chromosomes is not separated properly during meiosis, it can lead to an event called non-disjunction. Non-disjunction causes a chromosome to be missing or an extra chromosome present in the offspring. These mutations can lead to diseases like trisomy 21, also known as Down's syndrome, in which a person has three copies of chromosome 21.

Chromosomal Theory of Inheritance - Key Takeaways

  • The chromosomal theory of inheritance explains how genetic material is passed from parents to offspring through chromosomes.
  • Chromosomes are large structures of DNA found in the nucleus of each cell.
  • Each somatic cell has 46 chromosomes or 23 pairs of chromosomes.
  • Boveri and Sutton independently founded the chromosomal theory of inheritance in 1902.
  • The chromosome theory of inheritance follows three principles: fertilization, the law of segregation, and the law of independent assortment.
  • Unique examples of the chromosomal theory of inheritance include mutations, sex-linked genes, linked genes, and chromosomal disorders.

Chromosomal Theory of Inheritance

The chromosomal theory of inheritance explains how genetic material is passed from parents to offspring through chromosomes.  

Theodore Boveri and Walter Sutton independently developed the chromosome theory of inheritance in 1902 

Mendel's results can be explained by the action of chromosomes during meiosis 

The chromosomal theory of inheritance was accepted in 1902 following experiments performed independently by Theodore Boveri and Walter Sutton and 

Mendel's rules: The law of segregation and the law of independent assortment apply to genes on the chromosome 

Final Chromosomal Theory of Inheritance Quiz

Question

What is the chromosomal theory of inheritance?

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Answer

  • The chromosomal theory of inheritance explains how genetic material is passed from parents to offspring through chromosomes.  

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Who discovered the chromosomal theory of inheritance?

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Answer

Boveri and Sutton 

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How many chromosomes are found in a somatic cell?

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Answer

46 total or 23 pairs

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What are the three principles of the chromosomal theory of inheritance? 

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Answer

Fertilization

Law of independent assortment

Law of segregation 

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What is a gamete?

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Either a sperm or egg cell

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How many chromosomes does a gamete have?

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23

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How does the chromosomal theory of inheritance explain the law of segregation?

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One chromosome in a chromosome pair are passed onto a gamete during meiosis

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How does the chromosomal theory of inheritance explain the law of independent assortment?


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Answer

Genes located on different chromosomes do not influence the sorting of eacher

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How does the chromosomal theory of inheritance explain fertilization?


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One set of chromosomes from the sperm combines with one set of chromosomes from the egg to form 23 pairs of chromosomes 

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What are linked genes?

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Genes are that are located very close to each other on the same chromosomes 

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What are sex-linked genes

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Genes that are found on the X or Y-chromosome 

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What are mutations?

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Answer

Changes in the DNA sequence

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The observable trait is called 

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Phenotype

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The genetic combination of alleles is called: 

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Genotype

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What is the difference between a gene and an allele?

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A gene is a segment of DNA that determines a trait. An allele is one form of that gene.

For example, a gene can determine color, and alleles can be red, blue, white, black etc.  

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What is a chromosome disorder? 

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Answer

Chromosome disorders are changes to the number of chromosomes or chromosome sets leading to large-scale changes in the genome.

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Humans are ___ organisms.

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diploid 

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What is the difference between euploidy and aneuploidy?

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Euploidy is a change in the number of sets of chromosomes while aneuploidy is a change in the number of specific chromosomes. 

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What are the three trisomy disorders that can result in birth? 

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Trisomy 21 (Down syndrome)

Trisomy 18 (Edward syndrome

Trisomy 13 (Patau syndrome)

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Why do we only see three trisomies in humans? 

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Most trisomies will result in spontaneous abortion during pregnancy 

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What are the six most common classifications of sex chromosome disorders? 

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XXX females

XXY males (Klinefelter's syndrome) 

XX males

XYY malesXO (Turner's syndrome)

XY females

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What is the only monosomy that can result in live birth?

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Monosomy X

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Why do most XXX females not know of their genetic disorder? 

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Mild or absent symptoms 

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A haploid can be denoted as ___.

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N

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Most cells in the human are diploid which the exception of ____ cells.

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sex 

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Trisomy can be denoted as ____.

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2N + 1 

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Monosomy can be denoted as ____.


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2N - 1

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Triploids can be denoted as ___.

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3N

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____ results in sterile individuals.

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XXX

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Why are triploids typically sterile? 

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An uneven number of chromosome sets makes it difficult to equally divide set of chromosomes into daughter cells   

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What are sex-linked genes?

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Genes found on the X or Y chromosome 

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What are linked genes?

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Genes that are found close together on the same chromosome and tend to be inherited together

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What is the difference between parental and recombinant chromatids?

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  • Parental chromatids are chromatids that match the parent's alleles while recombinant chromatids are chromatids that are produced after a recombination event.

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What is recombination?

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An event during meiosis where homologous chromosomes are cut and exchange fragments of their DNA with each other to unlink linked genes 

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When does recombination occur?

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Prophase I

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If a two genes are found at the tip of two different chromosomes, they are ____.

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Unlinked

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If two genes are found together on either ends of the same chromosome, they are ____.

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Unlinked 

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If two genes are linked, offspring are much more likely to inherit ____ chromatids.

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Parental

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What might be a reason why people with red hair have light skin?

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The genes for red hair and light skin might be linked

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Red-green color blindness is a genetic disorder caused by a mutation for a photoreceptor gene found on the X chromosome. It is an X-linked recessive disorder. If a female carrier and a normal-visioned male have a child, what is the likelihood that the child will be colorblind? 


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25% 

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Red-green color blindness is a genetic disorder caused by a mutation for a photoreceptor gene found on the X chromosome. It is an X-linked recessive disorder. If a female carrier and a normal-visioned male have a male child, what is the likelihood that their male child will be colorblind? 


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50%

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Red-green color blindness is a genetic disorder caused by a mutation for a photoreceptor gene found on the X chromosome. It is an X-linked recessive disorder. If a female carrier and a normal-visioned male have a female child, what is the likelihood that the female child will be colorblind? 


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0%

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If two genes are unlinked, what is the frequency of the potential gametes? 

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Equal (25% each)


AB, Ab, aB, ab

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If two genes are linked, ___ gametes are more likely to be present than ___ gametes.

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parental, recombinant 

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True or False: A male with one copy of an X-linked recessive allele will express the recessive trait.  

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True 

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True or False: A female with one copy of an X-linked recessive allele will express the recessive trait.  

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False

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What is a sex-linked trait? 

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Sex-linked traits are traits that are determined by genes found on the X and Y chromosomes 

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What is an example of a sex-linked trait?  

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Red-green color blindness, hemophilia, and Fragile X syndrome are all examples of sex-linked traits. 

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How are sex-linked traits inherited? 


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Sex-linked traits are inherited in three ways: X-linked dominant, X-linked recessive, and Y-linked 

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Why are sex-linked traits more common in males? 


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Males are hemizygous for the X chromosome meaning that they only have one copy of the X chromosome. Therefore, regardless of whether a male inherits a dominant or recessive allele, they will express that trait. In contrast, females have two X chromosomes, therefore, a recessive allele can be masked by a dominant alelle.   

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