Sex Linked Traits and Pedigrees 1 2 3

Sex – Linked Traits and Pedigrees 1. 2. 3. Objective: Explain how sex-linked traits are inherited. Use a Punnett square to determine the probability of offspring inheriting a sex linked disorder. Demonstrate how to use pedigrees to trace the inheritance of a trait through several generations.

Gene Location and Pedigrees We refer to genes as either … ◦ Sex-linked (on the sex-determining chromosomes) X or Y ◦ Autosomal (not on the sex-linked chromosomes)

Sex Linkage Sex-linked traits: traits that are controlled by genes located on the sex chromosomes (X or Y) ex. X-linked recessive: ◦ red-green colour blindness in humans: a recessive allele located on the X gene ◦ XB represents normal vision (dominant trait) ◦ Xb represents colour blindness (recessive trait) More prevalent in males since females would have to inherit 2 recessive alleles to be red-green colour blind Since males inherit only one X chromosome they require only one recessive allele to be colour blind

X – Linked Alleles most sex-linked alleles are found only on the X chromosome the letter chosen to represent the trait is placed in the upper right hand corner e. g. colour blindness: ◦ XB represents normal vision (dominant trait) ◦ Xb represents colour blindness (recessive trait) usually sex-linked traits are recessive ◦ A female must receive two recessive alleles to express the recessive trait ◦ A male need only receive one recessive allele from his mother to express the recessive trait

Example… Look back at you previous slide. What would be the genotype of a colour blind woman? A colour blind man? A woman with who is a carrier? Carriers: heterozygous females who appear normal but who carry one recessive allele for an inherited X-linked genetic disorder

Genotype Female Male Phenotype XBXB (homozygous) Normal vision XBXb (heterozygous) Normal vision/ carrier Xb. Xb (homozygous) Colour blind XBy Normal vision X by Colour blind

Hemophilia Individuals that suffer from hemophilia bleed excessively when injured They possess an abnormal allele for clotting factor VIII in the blood The most seriously affected people may bleed to death from minor cuts or bruises There was a high incidence of hemophilia in the royal families of Europe due to intermarriage; Queen Victoria of England was a carrier of the hemophilia gene XHXh (the disease originated in her as a spontaneous mutation in her gene)

Example… Hemophilia (a disorder that prevents the blood from clotting properly) is a recessive disorder. It is represented with a ‘h’. (Xh) A woman with hemophilia and a man without hemophilia have children. What are the genotypes of each parent? Draw a Punnett Square to show the possible genotypes and phenotypes of the children. What do you notice about the male children?

Sex-Linked Disorders These disorders appear more often in males Why?

Ex. X-linked dominant

Pedigrees Experimental crosses are not done on humans Scientists study family trees or pedigrees to trace genetic diseases passed on from generation to generation Pedigrees reflect phenotypes

Human Pedigree This shows a brother and sister affected by a disease, along with their maternal grandmother

Goals of Pedigree Analysis A pedigree chart can be used to trace the passing of alleles from parents to offspring. It can also be used to determine the mode of inheritance (ie: sex-linked recessive or dominant or autosomal recessive or dominant)

Try and figure out the method of inheritance… Correct the pedigree to show any carriers. carrier X-linked recessive

Hemophilia individuals that suffer from hemophilia bleed excessively when injured they possess an abnormal allele for clotting factor VIII in the blood the most seriously affected people may bleed to death from minor cuts or bruises there was a high incidence of hemophilia in the royal families of Europe due to intermarriage; Queen Victoria of England was a carrier of the hemophilia gene XHXh (the disease originated in her as a spontaneous mutation in her gene)

Duchenne Muscular Dystrophy This involves the progressive weakening of muscles and the eventual loss of muscle tissue due to a missing protein It is evident early as the child has difficulty standing up and is usually wheelchair bound by age 12 Even normal breathing becomes difficult because the diaphragm and intercostal muscles are also affected The individual usually dies at around age 20 1: 3500 male babies are affected

Autosomal Inheritance We can also use Pedigrees to track the inheritance of traits carried on your autosomes. This means that both male and females have an equal chance of inheriting the disorder.

Try and figure out the method of inheritance… Autosomal recessive Which individuals are carriers?

Your work… 1) 2) 3) Complete questions… Sex-Linked Inheritance Questions Pedigree Questions Read pp. 197 – 201 #1 -5