Sex-linked Inheritance

Some Review: Autosomes: Non-sex chromosomes (#1-22 homologous pairs, 44 total chromosomes) Sex Chromosomes: Chromosome pair #23 Female XX Male XY

Sex Inheritance Sex Chromosomes carry Sex Traits and… …the Inheritance can be followed in a… …Punnett Square Gametes

Sex-linked traits Trait is carried on the X chromosome. The Y chromosome is too small to carry other traits besides the one determining the male sex characteristics.

Ex. In Drosophila (fruit flies): P: White-eyed male x Red-eyed female F1: All Red eyes, & Then for the F1 x F1 F2: ½ Red eyes, female ¼ Red eyes, male ¼ white eyes, males

**How do you know? **you know you are dealing with a sex-linked trait when mostly MALES exhibit the trait!!!**

XRXr = “carrier” female Carrier: Only females have two X chromosomes a carrier is heterozygous and carries a recessive trait but shows the dominant trait Therefore the recessive trait is masked and not exhibited but the gene is still there to be passed on. XRXr = “carrier” female

We can also use Punnett squares in questions concerning sex-linked traits

We can also use Punnett squares in questions concerning sex-linked traits Instead of using a lone letter to represent each allele, we connect it to the sex chromosome.

We can also use Punnett squares in questions concerning sex-linked traits Instead of using a lone letter to represent each allele, we connect it to the sex chromosome. Ex. XAXa or XAY

We can also use Punnett squares in questions concerning sex-linked traits Instead of using a lone letter to represent each allele, we connect it to the sex chromosome. Ex. XAXa or XAY The dominant allele is written as a capital letter and the recessive allele as a lowercase letter – just like in complete dominance. Ex. XAXa or XAXA or XAY or XaY The Y has no allele attached.

XA Xa XAXA XAXa XAY XaY XA Y ONE EXAMPLE

Human examples (notice that all are recessive traits): Color Blindness: Xc (XC = normal color vision) Ex. Two normal-visioned parents produce a color blind son. What are the genotypes of the parents? What are the chances of their next child being a color-blind daughter?

XC Y

XC XC Y

XC XcY XC Y

XC XCY XcY XC Y

XC XCXC XCY XcY XC Y

XC Xc XCXC XCY XcY XC Y

XC Xc XCXC XCXc XCY XcY XC Y

Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy?

Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? Xc Y

Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? XC Xc Xc Y

XC Xc XCXc XcXc XCY XcY Xc Y Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? XC Xc XCXc XcXc XCY XcY Xc Y

XC Xc XCXc XcXc XCY XcY Xc Y Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? XC Xc XCXc XcXc XCY XcY Xc Y

XC Xc XCXc XcXc XCY XcY Xc Y Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? XC Xc XCXc XcXc XCY XcY Xc Y

XC Xc XCXc XcXc XCY XcY Xc Y Ex. If a color blind man marries a woman whose father was color blind; what is the probability of their first child being a color blind boy? XC Xc XCXc XcXc XCY XcY Xc Y Color Blind Boy = 1/4

Hemophilia: Xh (XH = normal blood clotting) Ex. a man, whose father was a hemophiliac but who’s clotting time was normal, marries a woman with no record of hemophilia in her ancestry. What are the chances of hemophilia in their children?

XH XH XH Y Hemophilia: Xh (XH = normal blood clotting) Ex. a man, whose father was a hemophiliac but who’s clotting time was normal, marries a woman with no record of hemophilia in her ancestry. What are the chances of hemophilia in their children? XH XH XH 0% chance Y

Ex. A woman whose father was a hemophiliac, but who is not herself a “bleeder,” marries a normal man. What is the chance of hemophilia in their children?

Ex. A woman whose father was a hemophiliac, but who is not herself a “bleeder,” marries a normal man. What is the chance of hemophilia in their children? Hemophilia Children = 1/4

Ex. If a boy’s father has hemophilia and his mother’s cells have one gene for the disease, what is the chance the boy will inherit the disease?

Ex. If a boy’s father has hemophilia and his mother’s cells have one gene for the disease, what is the chance the boy will inherit the disease? XH Xh ½ or 50% chance XH Y

Ex. If a woman who is not a carrier of genes for hemophilia is married to a man who is a hemophiliac, what percentage of their male offspring could be expected to be hemophiliac?

Ex. If a woman who is not a carrier of genes for hemophilia is married to a man who is a hemophiliac, what percentage of their male offspring could be expected to be hemophiliac? 0% chance

Muscular Dystrophy: Xm (XM = normal) Mother is a carrier, father is normal. What is the chance for a MD child?

Muscular Dystrophy: Xm (XM = normal) Mother is a carrier, father is normal. XM Xm What is the chance for a MD child? XM Y

Muscular Dystrophy: Xm (XM = normal) Mother is a carrier, father is normal. M XM Xm XMXm XMY XmY What is the chance for a MD child? Xm Y

Muscular Dystrophy: Xm (XM = normal) Mother is a carrier, father is normal. One our of four children… …but one out of two sons. XM Xm XMXm XMY XmY XM Y