Sex-Linked Genes

Ms. KlinkhachornFebruary 14, 2011

AP Biology

Free Response Question Quick Write 15 minutes

• The unit of genetic organization in all living organisms is the chromosome.a) Describe the structure and function of the parts

of a eukaryotic chromosome. You may wish to include a diagram as part of your description

b) Describe the adaptive (evolutionary) significance of organizing genes into chromosomes

Using the Rubric

• Grade your neighbor’s essay and put a score on it

• Grade your own essay and put a score on it.

The Chromosomal Basis of Sex

• Two types of sex chromosomes– X and Y– Y is a lot smaller

• XX is female, XY is male– Females only pass on an x, but males can pass on

an x or a y chromosome• Your dad determines your sex• It is all chance. 50-50 chance

SRY gene

• Found on the Y chromosome• Stands for sex-determining region of Y– Responsible for the development of the testes• If no SRY gene, no testes ovaries (even if you have a

y chromosome)• Example of pleiotropy

Sex-linked Genes

• Genes located on sex chromosomes are called sex-linked genes– Specifically on the X chromosome (the y doesn’t

have many genes on it)– Most chromosomes on the sex chromosomes

have nothing to do with your actual sex• Mom can pass on sex-linked alleles to boys

and girls• Dad can only pass on sex-linked alleles to girls

Recessive Sex-Linked Alleles

• Girls will exhibit the phenotype if they are homozygous recessive (have 2 alleles)– If they are heterozygous they are a carrier

• Boys will exhibit the phenotype if they have even 1 allele

• Are more sex-linked disorders found in males or females?

Examples of Sex-Linked Disorders

• Colorblindness– Cannot see certain colors

• Duchenne muscular dystrophy– Progressive weakening of the muscle and loss of

coordination– Usually causes death in the 20s

• Hemophilia – Don’t have the gene necessary to make a protein

responsible for blood clotting

X-inactivation

• Females have 2 X chromosomes, but don’t want double the dosage– During embryonic development, a randomly

chosen X chromosome becomes inactivated in each cell• These stay inactive throughout the life cycles of the

cells and its descendents

• Process is called methylation– The inactive x chromosome becomes a Barr body

Linked Genes

• They are located near each other on the same chromosome and tend to be inherited together– The further apart the genes are located, the less

likely they are to be linked

Genetic Recombination

• Production of offspring with a new combination of genes inherited from parents

• Parental types = children that have the same phenotype as one of the parents

• Recombinants = children with phenotypes that are different from either parent

Quick Review

1. Name and describe the 3 recessive sex-linked diseases. List the phenotypes and the associated genotypes for one of the diseases.

2. What does methylation do and why is it necessary?

3. How is it possible to be genetically male but not look phenotypically male? Explain.

Chromosomal Abnormalities

• Nondisjunction– Homologous chromosomes do not separate

properly– Sister chromatids don’t separate properly

Nondisjunction

• Results in gametes receiving too many or too few chromosomes– Aneuploidy = incorrect number of chromosomes– Trisomy (3 instead of 2) vs monosomy (1 instead

of 2– Polyploidy = having more than 2 sets of

chromosomes• Sets represented by n• 3n, 4n – common in plants, not in animals

Alterations of Chromosome Structure

• Deletion• Duplication• Inversion• Translocation

Deletion

• Part of the chromosome gets lost during division

• Result?

Duplication

• A piece of a chromosome that broke off (deletion) ends up attached to its sister chromatid

• Result?• Duplication and deletion are the more

common chromosomal abnormalities

Inversion

• A chromosome fragment breaks off and then reattaches, but backwards

Translocation

• A deleted chromosomal fragment joins a nonhomologous chromosome

• If it’s reciprocal, the pieces will switch places– Can be nonreciprocal, but this isn’t as common

Related Disorders

• Down Syndrome• Klinefelter Syndrome• Turner Syndrome

• These are not hereditary diseases. They do not get passed down from one generation to the other.

Down Syndrome

• Aneuploid condition– Trisomy 21

• Characteristics: short stature, heart defects, mental retardation

• Chances of having a child with down syndrome increases with the mother’s age

Klinefelter Syndrome• Aneuploidy of the sex

chromosomes– XXY, occurs 1 in every 2000

births• Have male sex organs,

and even though the second X is inactivated, they might have larger than normal breasts and other more feminine characteristics

• Sterile

Turner Syndrome

• Monosomy X– Only monosomy that works in humans

• Occurs in 1/2000 births• Genotype: XO• Phenotype: female, but they are sterile– Their reproductive organs do not mature fully– Short stature– flat chest

Related Disorders

• Down Syndrome• Klinefelter Syndrome• Turner Syndrome

• These are not hereditary diseases. They do not get passed down from one generation to the other.

Unrelated Case Study: Tortoiseshell Cats

• Why is it that tortoiseshell cats are always female?

• Alleles for black and orange

• Mosaicism