Genetics: Sex-Linked Inheritance Bio 12

Review: Sex Chromosomes Humans have 23 pairs of chromosomes. 22 of them are called autosomal. The 23rd pair are the sex chromosomes. The sex chromosomes determine whether an offspring will be male or female. XX = Female XY = Male 2

Review: Females Have two x chromosomes (one from mother and one from father)

Review: Males Have one x chromosomes (from mother) and one y (from father)

What is sex-linked inheritance? Genes that are carried by either sex chromosome are said to be sex linked. Sex linked genes have inheritance patterns that differ from autosomal genes Since males and females differ in their sex chromosomes, inheritance patterns are different for males and females.

History….. The study of inheritance of genes located on sex chromosomes was pioneered by T. H. Morgan and his students at the beginning of the 20th century. He used Drosophila melanogaster (fruit flies) to study genetics Although Morgan studied fruit flies, the same genetic principles apply to humans.

More history….. Morgan noticed that chromosomes of male and female fruit flies were slightly different He is credited as the first person to see the physical difference in the x and y chromosomes!

More history….. Knowing red eyes are dominant and white eyes are recessive, Morgan crossed whited eyed with red eyed F1’s were what he expected 3 red: 1 white But in F2, he found all the white eyed flies where male! He predicted that the eye colour was connected with the sex of the fruit fly!

Genotype notation: Use X or Y to represent sex chromosome and capital or lowercase letters to represent dominant or recessive allele Examples: XG XG XG Xg Xg Xg XG Y Xg Y

X-linked X-linked diseases are those for which the gene is present on the X chromosome. Because of this, males and females show different patterns of inheritance and severity of disease. There are both dominant and recessive X-linked diseases

X-linked Characteristics: X-linked genes are never passed from father to son. Males are never carriers – if they have a mutated gene on the X chromosome, it will be expressed …. Why? The Y chromosome is the only sex chromosome that passes from father to son……does not pass on an X!

X-linked dominant X-linked dominant diseases are those that are expressed in females when only a single copy of the mutated gene is present. Very few X-linked dominant diseases have been identified (e.g. hypophosphatemic rickets, Alport syndrome, diabetes insipidus)

X-linked dominant diseases: Affected females produce 50% normal and 50% affected offspring…….. heterozygous Females are more likely to be affected. Since females have 2 X chromosomes, they have 2 “chances” to inherit the mutated allele.

The pattern for the pedigree of X-linked dominant inheritance: Note: If father is affected, all daughters will be affected

X-linked recessive X-linked recessive diseases are those in which a female must have two copies of the mutant allele in order for the mutant phenotype to develop. Trait skips generations Affected fathers DO NOT pass to their sons Males are more often affected than females Includes: color blindness, hemophilia, and Duchenne muscular dystrophy.

The pattern for the pedigree of X-linked recessive inheritance:

Y…. In humans there are very few traits that are sex linked on the “Y” chromosomes These traits are only expressed in males Transmitted from father to son Example: gene SRY (triggers testis development ) Note: We will not be doing any y-linked genetics problems in Bio 12!

To do: X-linked Genes WS Page 167 #6, 7