7 Since both of a female’s sex chromosomes are X, all eggs carry one X chromosome Males have two different sex chromosomes, so sperm cells will either carry an X or a Y chromosome
8 Sex-Linked GenesSome human traits occur more often in one gender than the other.Sex-Linked Genes: Genes on the X and Y chromosomes, whose alleles are passed from parent to offspring on sex chromosomes
9 What is a sex-linked trait? Sex-linked traits are due to genes located on sex chromosomes.As we know, male sex chromosomes are XY while female sex chromosomes are XX.The X chromosome contains over 1000 genes while the Y chromosome contains as few as 26. Therefore, many sex-linked traits are discussed in terms of the X-chromosomes.
10 CarriersBecause females have two copies of the X chromosome, it is possible to have certain traits “hidden” by a dominant copy.However, because males only have one X chromosome, the observable phenotype is obvious and identifies the genotype.
11 CarriersWhen a female contains a recessive allele that is hidden by the dominant allele, we call them carriers.A carrier maintains the ability to pass on a trait even if they do not express/show it.
12 In females, a dominant allele on one X chromosome will mask a recessive allele on the other X chromosome.
13 In males, there is no matching allele on the X and Y chromosome In males, there is no matching allele on the X and Y chromosome. As a result, any allele on the X chromosome will produce the trait in a malewho inherits it.
14 Because males have only one X chromosome, males are more likely than females to have a sex-linked trait that is controlled by a recessiveallele.
15 Example of sex-linked trait controlled by a recessive allele: red-green Color Blindness affects more males thanfemales.
16 Examples of X-linked genes Other than determining sex, genes on the X chromosome are responsible for traits. Some examples are:HemophiliaRed-green color blindnessHigh blood pressureMuscular dystrophyFragile-X syndrome
17 Recessive LethalsIn some rare instances, recessive traits are lethal-- meaning the organism is born very weak and sickly or dies not long after birth.When looking at non-sex linked chromosomes, we can apply standard Punnett Square probability.However, when looking at sex linked chromosomes we see that males have a much higher tendency for recessive lethals. Why?