Chromosomes, Mapping, and the Meiosis-Inheritance Connection Chapter 13

I.Introduction A.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 determination a)XX = female b)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 genetics A.Difficult because 1.Long generations (25 yrs) 2.Single births 3.Ethical concerns

B.Methods for studying humans 1.Pedigrees: record that shows how a trait is inherited within a family

9

2.Population sampling 1.Selecting a small # of people to represent an entire population 2.Must be a random sample 3.Ex: through random sampling, researchers discovered that 65% of people in US taste PTC, 35% cannot

3.Identical twin studies a)Identical genetic codes b)Separated at birth: study which traits are genetic(Nature) & which are environmental (Nurture)

13 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

14

III.Inheritance of human traits A.Dominant traits 1.Polydactyly - extra fingers & toes 2.Dwarfism - small size 3.Curly hair 4.Huntington disease - nervous disorder 5.Piebaldness - white patches of hair

Vitiligo=AutoImmune Piebaldness=Genetic

B.Recessive traits 1.Straight hair 2.Freckles 3.Albino 4.Cystic fibrosis - lung disorder 5.Deafness 6.PKU - nervous disorder

19 Odds of having a child with the disease?

C.Codominance 1.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)

21 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.

22

F.Sex-linked traits 1.Genes located on X chromosome 2.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

27

7.Color vision a)X C =Color Vision, X c =colorblind b)Males - 8% colorblind (X c Y) c)Females - 1% colorblind (X c X c ) 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 disorders A.Nondisjunction: failure of chromosomes to separate during meiosis 1.Normal = 23 chromosomes 2.Abnormal = 22 or 24

32

3.Monosomy: too few chromosomes, one chromosome left unpaired a)Turner syndrome: female with only 1 X chromosome b)Short stature, sterile

4.Trisomy: too many chromosomes, extra copy of one a)Down’s syndrome: trisomy 21 b)1 in 1400 when mother under age 25, 1 in 100 by age 40

35 Human Genetic Disorders Nondisjunction of sex chromosomes can result in: XXX triple-X females XXY males (Klinefelter syndrome) XO females (Turner syndrome) OY nonviable zygotes XYY males (Jacob syndrome)

B.Detecting genetic disorders 1.Physical characteristics - features, disabilities 2.Ultrasound - pictures of baby in utero using sound waves 3.Amniocentesis - sample of fluid from around baby, can examine baby’s cells

39

40

4.Karyotype: picture of a person’s cromosomes taken from any cells -- blood in adults, amnio for baby Normal Trisomy 21

42