Chromosomes, Mapping, and the Meiosis-Inheritance Connection

Chapter 13

HUMAN GENETICS

I. IntroductionA.46 chromosomes in each

cell (23 pairs)B.Autosomes: all

chromosomes except sex chromosomes (22 pairs)

C.Sex chromosomes: determine gender (1 pair)

1.Sex determinationa)XX = femaleb)XY = male

2.During meiosis, each egg from the female gets an X; 1/2 male sperm get X, 1/2 get Y

3.Males determine gender of offspring!a)Except in birds and reptiles

where female is XY and male is XX

II. Studying human geneticsA.Difficult because

1. Long generations (25 yrs)2. Single births3. Ethical concerns

B.Methods for studying humans1.Pedigrees: record that shows

how a trait is inherited within a family

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2. Population sampling1. Selecting a small # of people to

represent an entire population2. Must be a random sample3. Ex: through random sampling,

researchers discovered that 65% of people in US taste PTC, 35% cannot

3. Identical twin studiesa) Identical genetic codesb)Separated at birth: study which

traits are genetic(Nature) & which are environmental (Nurture)

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Human Genetic Disorders

Some human genetic disorders are caused by altered proteins.

the altered protein is encoded by a mutated DNA sequence

the altered protein does not function correctly, causing a change to the phenotype

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III. Inheritance of human traitsA. Dominant traits

1. Polydactyly - extra fingers & toes

2. Dwarfism - small size3. Curly hair4. Huntington disease - nervous

disorder5. Piebaldness - white patches of

hair

Vitiligo=AutoImmune

Piebaldness=Genetic

B.Recessive traits1.Straight hair2.Freckles3.Albino4.Cystic fibrosis - lung disorder5.Deafness6.PKU - nervous disorder

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Odds of having a child with the disease?

C.Codominance1.Sickle cell anemia - irregularly

shaped red blood cells• O allele = healthy(HH)• 1 allele = mild case(HS),

resistant to malaria• 2 alleles = severe case(SS)

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Sex Chromosomes

In each female cell, 1 X chromosome is inactivated and is highly condensed into a Barr body.

Females heterozygous for genes on the X chromosome are genetic mosaics.

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F. Sex-linked traits1. Genes located on X chromosome2. Males only have one X, so only

have one allele controlling these genes

3. Males more likely to have disorders that are sex-linked because they only need 1 recessive allele

4. Carrier: someone who has one allele for a disorder -- does not have the disease but can pass it on to children

5. Only women can be carriers for sex-linked traits

6. Examples: hemophilia, muscular dystrophy, colorblindness

Hemophilia in the Royal Families of Europe

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7. Color visiona)XC=Color Vision, Xc=colorblindb)Males - 8% colorblind (XcY)c) Females - 1% colorblind (XcXc)

Normal

Red-Green

Total

What are the chances for a healthy dad and a mom who is a carrier for colorblindness to have a child that

is colorblind?

IV. Genetic disordersA. Nondisjunction: failure of

chromosomes to separate during meiosis

1. Normal = 23 chromosomes2. Abnormal = 22 or 24

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3. Monosomy: too few chromosomes, one chromosome left unpaireda)Turner syndrome: female with

only 1 X chromosomeb)Short stature, sterile

4. Trisomy: too many chromosomes, extra copy of onea)Down’s syndrome: trisomy 21b)1 in 1400 when mother under age

25, 1 in 100 by age 40

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Human Genetic Disorders

Nondisjunction of sex chromosomes can result in:

XXX triple-X femalesXXY males (Klinefelter syndrome)XO females (Turner syndrome)OY nonviable zygotesXYY males (Jacob syndrome)

B.Detecting genetic disorders1. Physical characteristics - features,

disabilities2. Ultrasound - pictures of baby in

utero using sound waves3. Amniocentesis - sample of fluid from

around baby, can examine baby’s cells

http://www.biology.iupui.edu/biocourses/n100/2k4csomaldisordersnotes.html

http://www.katie.com/babyblog/archives/000538.html

http://www.katie.com/babyblog/archives/000538.html

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4.Karyotype: picture of a person’s cromosomes • taken from any cells -- blood

in adults, amnio for baby

Normal Trisomy 21

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