Biology 7.4 and 7.5

Sex Determination The X & Y chromosomes are involved in producing the sex of an individual through a process called sex determination. In humans, sex is determined by the male since males produce sperm that carry either an X or a Y chromosome.

The X and Y chromosomes The Y chromosome is smaller than the X chromosome and carries fewer genes.

Sex Determination Birds: The male is ZZ The female is ZW The female determines the sex of the offspring.

Sex Determination Sex can be determined by the incubation temperature of the egg. For example, in turtles, females are born if the eggs are very hot, and males are born at a cooler temperature.

Sex Determination Sex depends on fertilization. If the eggs are fertilized, females are born. If the eggs are not fertilized, males are born.

Sex Determination Some species of bony fish change their sex after maturation. All individuals become female unless they are deflected from that pathway by social signal such as dominance interactions.

Sex Determination The nematode C. elegans can either be male or have both male and female reproductive organs. Individuals with male and female reproductive organs are hermaphrodites.

Questions How is sex determined in humans? How is sex determined in birds? How is sex determined in turtles? How is sex determined in wasps, ants, and bees? How is sex determined in bony fish? What are the possible sexes of C. elegans?

X-linked genes versus Y-linked genes Genes on the X or Y chromosomes are called sex linked genes because biological sex is inherited along with genes on the X or Y chromosome. Sex linked genes on the X chromosomes are X linked genes. Sex linked genes on the Y chromosome are Y linked genes. The Y chromosome carries very little genetic information because it is smaller than the X chromosome.

X linked genes X linked genes are located on the X chromosome. Males inherit X linked genes only from their mothers. Males are more likely to suffer from diseases caused by recessive alleles on the X chromosome. A carrier of a recessively inherited trait has one copy of the recessive allele and one copy of the normal allele and will not exhibit symptoms of the disease. Only females can be carriers of X linked recessive traits. Both males and females can be carriers of non sex- linked, autosomal traits.

Example: Hemophilia A recessive trait that is carried on the X chromosome. Hemophilia is a genetic disorder which causes poor blood clotting and coagulation. Because it is X linked it is more often found in males. Females can carry one affected allele (carrier) but not show any symptoms.

Hemophilia Hemophilia is on the X chromosome, therefore we will use the letter X. XH is considered normal and Xh is afflicted A normal female can be XHXH or XHXh. A female with the disease is XhXh A normal male will be XHY A male with the disease will be XhY

Hemophilia Genetics Cross 1

Hemophilia Genetics Cross 2

Can you see it? 4% of males affected Cannot see red or green. This is red-green color blindness.

Sex Linked Trait: Color Blindness Color Blind: Male: XrY Female: XrXr Normal: Male: XRY Female: XRXR Female Carrier: XRXr

Sex Linked Traits Worksheet

Muscular Dystrophy Progressive, fatal X linked to disease of muscle wasting the affects approximately 1 in 3500 males. Muscle wasting starts between 1 and 12 years of age, and by age 12, affected boys are in wheelchairs.

X inactivation X inactivation guarantees that all females actually receive only one dose of the proteins produced by genes on the X chromosomes. This happens in the embryo when it implants in the uterus. One chromosome is inactivated when a string of RNA is wrapped around it. Inactivation is irreversible and, as such, is inherited during cell replication. Once an X chromosome is inactivated in a cell, all descendants of that cell continue inactivating the same chromosome.

X inactivation and patchy gene expression

Tortoise Shell Almost always happens in females. Can happen in males via nondisjunction if he is XXY.

Y linked genes Y linked genes are passed from fathers to sons. The SRY (sex determining region of the Y chromosome) gene is on the Y chromosome. The SRY gene develops the testes and male sexual characteristics. Genes other than SRY, that are on the Y chromosome, code for proteins that are unique to males but are not expressed unless testes develop.

Pedigrees A pedigree is a family tree that follows the inheritance of a genetic trait for many generations of relatives. Pedigrees are used in studying human genetics. Scientists can use pedigrees to determine whether a trait is inherited as autosomal dominant or recessive or as sex linked recessive.

Dominant trait: Polydactyly

Recessive trait: Attached Earlobe

Sex-linked trait: Muscular Dystrophy

Origin and Inheritance of the Hemophilia Allele

Pedigrees Worksheet