Ch.20 Chromosomes and Human Genetics

BIOL 107 Exam 5

How are genes usually sorted during gamete formation?

They are sorted independently (independent assortment)

Why do relatives differ genetically?

Independent assortment

What is linkage?

Distance between genes on a chromosome

Closely linked genes ______ separate during ______ _____.

rarely, crossing over

Dinstant genes on the same chromosome…

Are more likely to be separated by crossing over

What are the steps in preparing a karyotype?

1. Culture cells and stimulate

2. Add colchicine, stops at metaphase

3. Centrifuge to collect cells

4. Transfer cells to saline —→ swell, chrom. separate

5. Spread on slide, fix & stain

6. Photos through microscope

7. Cut and arrange by size (21 smallest!!)

Genes in X vs. Y chromosome

X: 1,400 genes

Y: 100 genes

How is sex determined?

Egg ALWAYS contributes an X, Sperm determines sex

What is the SRY gene on Y chromosome?

It acts as the “master switch” for male development

SRY present—→ testes

NO SRY present—→ovaries

Which chromosome carries many non sex related genes?

X chromosome

Which chromosome mostly carries genes related to male sex development?

Y chromosome

What is the only way Y-linked traits can be aquired?

Father to son

What is X chromosome inactivation?

Ensures dosage compensation in females (F&M prod similar amounts of X-linked gene products)

Early in development, one _ chromosome is _______ in each cell

X , inactivated (either maternal or paternal X)

What do inactivated X condense into?

A Barr body

All daughter cells inherit….

the same X inactivated

What are sex-influenced traits?

Autosomal traits influenced by sex (can appear in both sexes but expressed differently)

What are examples of sex-influence traits?

1. Voice pitch

2. Muscle mass

3. Acne severity

What are sex-limited traits?

Autosomal traits only visible in one sex (both sexes are carriers but only visible in 1 sex)

What are some examples of sex-limited traits?

1. Beard growth in males

2. Breast development in females

What are pedigrees used for?

They help identlfy likely inheritance patterns and likelyhood of transmitting certain genes (mainly genetic disorders)

What is a genetic abnormality?

Uncommon form of a trait; not necessarily harmful

What is a genetic disorder?

Inherited condition causing mild to severe health problems

What are some examples of genetic abnormalities?

1. Lactose persistence

2. Red hair

3. Tounge rolling

4. Polydactyly

What are autosomal recessive inheritance?

Recessive allele located on an autosome

What is autosomal dominant inheritance?

It is the dominant allele on an autosome, trait typically appears in every generation

Some dominant alleles persist because…

1. They arise by new mutations

2. Do not prevent reproduction

3. Symptoms appear after reproduction age

What is X-linked recessive inheritance?

Recessive allele located on the X-chromosome (heterozygote females are carriers)

Why are males more affected for X-linked recessive inheritance?

They only have one X, so recessive allele is always expressed

What are some structural changes that chromosomes can undergo?

1. Deletions

2. Duplications

3. Translocations

4. Inversions

Structural changes can disrupt normal gene function

What is non-disjunction?

Failure of chromosomes pairs to seprate during meiosis (most common) or mitosis

What is the result of non-disjunction?

Gametes/cells with too many or too few chromosomes

Results of too many or too few chromosomes

Extra: Trisomy (2n+1) MOST COMMON & can be seen in a karyotype

Less: Monosomy (2n-1)

What is autosomal aneuploidies?

Genetic condition where there is an extra or missing copy of one or more of the non-sex chromosomes

Only 3 autosomal trisomies

1. Trisomy 21- Down Syndrome (1/700)

2. Trisomy 18- Edwards Syndrome (1-5,000)

3. Trisomy 13- Patau Syndrome (1-15,000)

Monosomy of autosomes (complete loss of one copy of non-sex chrom)…

Always embryonic lethal

Where do most sex chromosome abnormalities arise from?

Non-disjunction during gamete formation

Examples of non-disjuntion of sex chrom

1. Turner Syndrome (XO)—→F. no puberty, premature aging

2. Triple X Syndrome (XXX)—→F. 2 Xs become Barr bodies

3. Klinefelter Syndrome (XXY)—→M. Low fertility, testosterone may help

4. Jacob Syndrome (XYY)—→M. taller & aggresive

Non-disjunction generally ___ ______ than autosomal non-disjunction

less disruptive