SEX-LINKED TRAITS Solving pedigrees for traits with genes located on the X and Y chromosome

AUTOSOMES VS. SEX CHROMOSOMES Humans have 23 pairs of chromosomes Humans have 23 pairs of chromosomes 22 pairs of chromosomes are not related to gender (male or female). These are called AUTOSOMES 22 pairs of chromosomes are not related to gender (male or female). These are called AUTOSOMES 1 pair of chromosomes determines gender. These are called SEX CHROMOSOMES 1 pair of chromosomes determines gender. These are called SEX CHROMOSOMES

KARYOTYPE chart of homologous chromosomes (XX)

KARYOTYPE chart of homologous chromosomes (XY)

AUTOSOMAL TRAITS Most of the traits in humans are controlled by genes on the autosomes. Most of the traits in humans are controlled by genes on the autosomes. Autosomal traits are inherited the same way by both males and females. Autosomal traits are inherited the same way by both males and females. All of the traits we have studied so far have been autosomal – they are the simplest type of genetics problems to solve All of the traits we have studied so far have been autosomal – they are the simplest type of genetics problems to solve

SEX CHROMOSOMES The sex chromosomes determine gender The sex chromosomes determine gender The sex chromosomes are also called the X and Y chromosome The sex chromosomes are also called the X and Y chromosome Females are XX Females are XX Males are XY Males are XY The Y chromosome is much smaller, and does not have the same genes as the X The Y chromosome is much smaller, and does not have the same genes as the X Most “sex-linked” traits are “X linked” Most “sex-linked” traits are “X linked”

INHERITANCE OF SEX CHROMOSOMES Since Females are XX Since Females are XX –They inherited an X chromosome from both of their parents –They will only produce eggs with X chromosomes –All of their children will inherit one or the other of her X chromosomes (with equal probability) – but all of her children, both boys and girls, will inherit an X chromosome from her

INHERITANCE OF SEX CHROMOSOMES Since males are XY Since males are XY –They inherited the Y from their father must be – their mother did not have a Y chromosome because she is female must be – their mother did not have a Y chromosome because she is female –They inherited the X from their mother process of elimination – if the Y came from Dad, the X came from Mom process of elimination – if the Y came from Dad, the X came from Mom

Females vs. Males X-Linked Traits Since females are XX Since females are XX –They have 2 alleles for the trait. It works out exactly like autosomal traits Homozygous dominant, heterozygous, and homozygous recessive are all possible Homozygous dominant, heterozygous, and homozygous recessive are all possible Since males are XY Since males are XY –They only have one allele for the trait. There is no corresponding allele on the Y chromosome –Whatever alleles males have on the X chromosome will be expressed.

Eye Color in Fruit Flies White eyes in fruit flies is X linked recessive (the normal “wild type” eye color is red)

Predict the offspring of a cross between a white eyed male and a purebred red eyed female

Predict the offspring of a cross between a white eyed male and a red eyed female All offspring will have red eyes All offspring will have red eyes Females will all be carriers Females will all be carriers

Predict the offspring of a cross between a normal male and a heterozygous female

All female offspring will have red eyes, but half of them will be carriers All female offspring will have red eyes, but half of them will be carriers Half of the males will be normal, and half will have white eyes Half of the males will be normal, and half will have white eyes

Colorblindness: X linked recessive People affected by red-green colorblindness are not able to distinguish red from green People affected by red-green colorblindness are not able to distinguish red from green The pictures above are used to test for colorblindness The pictures above are used to test for colorblindness

Colorblindness: Genotypes and Phenotypes Females: Females: –X N X N normal vision, not a carrier –X N X n normal vision, carrier –X n X n colorblind Males Males –X N Ynormal vision –X n Ycolorblind

Colorblindness: Pedigree solving All males are XY All males are XY All females are XX All females are XX Males inherit X from mother and Y from father. Males inherit X from mother and Y from father. Males give X to daughters and Y to sons Males give X to daughters and Y to sons Trace the X and Y through the generations Trace the X and Y through the generations

Colorblindness: Pedigree solving All males are XY All males are XY All females are XX All females are XX

Colorblindness: Pedigree solving #3 is colorblind #3 is colorblind Colorblindness is X linked recessive Colorblindness is X linked recessive #3 must have a recessive allele for colorblindness on his X chromosome #3 must have a recessive allele for colorblindness on his X chromosome All other males are normal, and must have dominant alleles for color vision on their X chromosomes All other males are normal, and must have dominant alleles for color vision on their X chromosomes

Colorblindness: Pedigree solving All the females in this pedigree have normal color vision, so all of them must have at least one dominant allele (maybe both) for normal color vision on an X chromosome All the females in this pedigree have normal color vision, so all of them must have at least one dominant allele (maybe both) for normal color vision on an X chromosome

Colorblindness: Pedigree solving #3 inherited his Y chromosome from his father (his mother doesn’t have one because she’s female) #3 inherited his Y chromosome from his father (his mother doesn’t have one because she’s female) He inherited his X chromosome from his mother He inherited his X chromosome from his mother #2 (mother of #3) is a carrier #2 (mother of #3) is a carrier

Colorblindness: Pedigree solving #5 is the daughter of #3 (colorblind male) and #4 (normal female) #5 is the daughter of #3 (colorblind male) and #4 (normal female) #5 has 2 X chromosomes, one from her Mom and one from her Dad #5 has 2 X chromosomes, one from her Mom and one from her Dad #5 is a carrier because she inherited her Dad’s X chromosome #5 is a carrier because she inherited her Dad’s X chromosome

Colorblindness: Pedigree solving The only person left is #4 – a normal female The only person left is #4 – a normal female Without knowing more about her parents, we can’t determine if she is a carrier or not. Her genotype is unknown Without knowing more about her parents, we can’t determine if she is a carrier or not. Her genotype is unknown