Autosomal Traits vs. Sex-linked Traits
Somatic cells, all the cells of the body besides gametes, contain 23 pairs of chromosomes. The first 22 pairs are autosomes because they are the same in males and females. For example, the two prominent genes that code for eye color (at least 16 genes code for eye color) are on Chromosome 15. Since the 15th pair of chromosomes are autosomes, both sexes have a pair of alleles for the two eye color genes. However, this is not the case for the 23rd pair of chromosomes. On the 23rd pair, females have a pair of alleles for each gene, but males only have one allele.
What? How?
The 23rd pair is the sex chromosomes that determine an individual’s genetic and biological sex. Females have a homologous pair of X chromosomes, which they receive from both parents. Males have a nonhomologous pair of chromosomes. Males receive an X chromosome from their mom and a Y chromosome from their dad. The X and Y chromosomes have different genes; therefore, males only have one allele to a sex-linked trait. (Sex-linked traits are also called X-linked traits since the X chromosome is the only sex chromosome males and females share.)
chapter 24: sex-linked traits
table of contents show
Autosomal Traits vs. Sex-linked Traits
Somatic cells, all the cells of the body besides gametes, contain 23 pairs of chromosomes. The first 22 pairs are autosomes because they are the same in males and females. For example, the two prominent genes that code for eye color (at least 16 genes code for eye color) are on Chromosome 15. Since the 15th pair of chromosomes are autosomes, both sexes have a pair of alleles for the two eye color genes. However, this is not the case for the 23rd pair of chromosomes. On the 23rd pair, females have a pair of alleles for each gene, but males only have a single allele.
What? How?
Well, the 23rd pair is the sex chromosomes that determine an individual’s genetic, and biological sex. Females have a homologous pair of X chromosomes, which they received from both parents. Males have a nonhomologous pair of chromosomes. Males receive an X chromosome from their mom and a Y chromosome from their dad. The X and Y chromosomes have different genes; therefore, males only have one allele to a sex-linked trait. (Sex-linked traits are also called X-linked traits since the X chromosome is the only sex chromosome males and females share.)
Biological Sex is Complex
The complexity of biological sex is determined by more than our chromosomes, and the exact causes are still not entirely understood. Science used to think that all individuals with XX are females and XY are males. However, genetic and biological research is discovering that a person’s biological sex is a product of genetic, biological, and environmental factors. A person can be XX and be a male, and XY and be a female. However, for this chapter, we focus on the genotype of XX being female and XY being male, but be aware that chromosomes are not the only factor determining our sex.
Sex-linked Traits
Recessive sex-linked traits are traits on the X chromosome, not the Y chromosome. Since the X chromosome is the only sex chromosome males and females share, alleles reside on the X chromosome. Females have two X chromosomes to be carriers of a recessive sex-linked trait. In other words, they can be heterozygous for a sex-linked trait. Also, females will only have a recessive sex-linked trait if they receive a recessive allele from mom and dad.
| Genotype | Phenotype |
|---|---|
| XA XA | No Hemophilia |
| XA Xa | No Hemophilia |
| Xa Xa | Hemophilia |
Males have one X chromosome, so they cannot be carriers of a sex-linked trait. Males receive the X chromosome from their mom, and that chromosome will have either a dominant or recessive allele. Thus, it takes one recessive allele for males to have a recessive sex-link trait. Because males receive one sex-linked allele from mom and none from dad, males cannot be carriers (heterozygous) of a sex-linked trait.
Males with hemophilia receive a recessive allele from ONLY mom.
| Genotype | Phenotype |
|---|---|
| XA Y | No Hemophilia |
| Xa Y | Hemophilia |
