Chapter 12: Patterns of Heredity and Human Genetics
Determining Genotypes
Testcross- a cross of an individual of unknown genotype with an indiv. if
known genotype in order to determine the unknown genotype.
Usually, the test organism is homozygous recessive
P1 Unknown (red flower) x rr (white flower)
F1 50% red flowers and 50% white flowers
What is the genotype of the unknown?
What if F1 had been 100% red flowers?
Selective Breeding
Breed- a selected group of organisms within a species that has been bred
for particular characteristics (ex. plants, horses, dogs)
Inbreeding- mating between closely related organisms
ensures offspring are homozygous for most traits
Hybrids- the offspring of 2 closely related species or heterozygous
varieties
often larger and stronger than parents
Pedigree- maps inheritance through a family
unaffected affected
male
female
mated
known heterozygous ("carrier")
death
Generation denoted by roman numerals
Individuals- arabic numbers
Simple Recessive Heredity
Most genetic disorders are recessive
Specific diseases tend to be specific to certain ethnic groups
Cystic Fibrosis (CF)- afflicts mainly white Americans
1:20 individuals carrier
1:2000 individuals are affected (homozygous)
CFTR - Cystic Fibrosis Transmembrane Receptor - membrane
protein involved in active transport of sodium ions
DF508- defective in folding, does not get to membrane
Results in thickened mucuous in lungs and digestive tract
Tay-Sachs - afflicts mainly Amish and Eastern European Jews
b-hexosaminidase A - enzyme responsible for break-down of lipid
produced/ stored in CNS is defective, so lipid accumulates
Results in CNS damage
Phenylketonuria (PKU)- afflicts mainly Swedes and Norwegians
Enzyme responsible for breaking down amino acid
phenylalanine into tyrosine is defective, phenylalanine
accumulates
Results in CNS damage, mental retardation
Post-natal test can detect PKU so child can be put on
special diet until brain fully develops. Homozygous females must follow diet during pregnancy, or fetus will be damaged
Other recessive diseases:
Albinism - lack tyrosinase, enzyme required to produce melanin
(1:38000 caucasian, 1:22000 African Americans)
galactosemia- lack galactokinase, can't metabolize lactose - cataracts,
mental retardation, poor growth, death from liver damage
Metachromatic leukodystrophy- arylsulfatase deficiency-build up
negatively charged lipids in CNS-degenerative: loss of intelligence
-> mental retardation
Simple Dominant Heredity
Tongue rolling, Hapsburg lip, hanging earlobes, hitchhikers thumb, almond eyes, full lips
Huntington's Disease
Brain degeneration
Onset between ages 30 and 50
Often passed to children before individual knows he/she is afflicted
There is now a genetic test
Every child of afflicted individual has 50% chance
Achondroplasia
Dwarfism- mutation of the transmembrane domain of the
Fibroblast Growth Factor Receptor -> inhibition of linear bone
growth
Marfan syndrome-
weak connective tissues in heart, skeletal system, eyes
defective fibrillin gene
1:5000
When Heredity Follows Different Rules
Other patterns of inheritance besides simple mendelian inheritance
Incomplete Dominance
Heterozygous individuals have intermediate phenotype
Capital and Capital' indicates incomplete dominance
P1 Red Snapdragon RR x R'R' White Snapdragon
F1 RR' Pink Snapdragons
F2 1 RR Red 2 R'R and RR' Pink 1 R'R' White
Codominance
Phenotype of both homozygous indiv. expressed in
heterozygote
Both alleles are capital b/c both dominant
Chicken feather colors
P1 Black BB x WW White
F1 Black/White BW
Multiple phenotypes from multiple alleles
Sometimes a gene has more than 2 alleles
Think: a new allele forms each time there is a mutation?
Only 2 alleles per diploid cell, but can be many in a
population
Pigeons- 3 alleles for feather color
Pleiotropy- single gene has multiple effects
gene that controls fur pigmentation in Siamese cats also
influences connections between eyes and brain (therefore,
cats with mutation in that gene are crosseyed and
abnormally pigmented)
Sex Determination
humans have 23 pairs of chromosomes
22 pairs are homologous (autosomes)
23rd pair differs between males and females
female XX
male XY (nonhomologous chromosomes)
Sex-Linked Inheritance
Traits controlled by genes on sex chromosomes
Alleles are written as superscripts on X or Y
Y does NOT have the same genes as X
1910- Thomas Hunt Morgan working w/ Drosophila melanogaster
Noticed a male fly w/ white eyes (usually red eyes)
P1 XrY x XRXR
F1 XrXR and XRY all red eyes (even males)
F2 XrXR XRXR XrY XRY
only males had white eyes
According to Morgan, red is dominant to white, and the gene for eyecolor is on X not Y. In males, there is only one X, so if it carries recessive white allele, that is what shows
How can males survive with only one X in all of their cells when females have 2?
Barr bodies!- in female somatic cells, one X shrivels and the genes are unexpressed. That means that one cell can display different X-linked traits than other cells in the same individual. Calico cats...
Polygenic Inheritance
more than one gene controls trait, on same or different
chromosome
skin color, corn cob length
Capital- adds to trait, lowercase- no effect
No dominance- all heterozygotes have intermediate
phenotype
Plant Height:
aabbcc 4 cm AABBCC 16 cm
P1 aabbcc x AABBCC
F1 AaBbCc 10 cm
Influence of Factors other than Genes
Genes = potential for expression
Environmental Factors
Temp, light, nutrition, chemicals- can play a role in gene exp
Serratia marcescens - bacteria that grows red at 25°C and
white at 37°C
On trees- leaf size depends on amount of light
Internal Factors
Age, Hormones, structural differences (btw male and
female)
horn size in sheep, deer
male pattern baldness
peacock vs peahen color
Complex Inheritance of Human Genetic Traits
Codominance
sickle-celled anemia- Black Americans of African descent
White Mediterranean Americans
1:12 African Americans is a carrier
Homozygotes- 1 amino acid change in hemoglobin causes the
protein to crystallize, changes the shape of RBC
Slows blood flow, blocks vessels, tissue damage
Sickle cells have shorter life span, so afflicted indiv.
have anemia from lower RBC counts
Heterozygotes- make sickle and normal, usually ok, except
in situation of limited oxygen
Multiple Alleles-
Blood Type controlled by gene I- 3 alleles
blood type determined by proteins expressed on surface of
cells
Genotype Phenotype (blood type)
IAIA or IAi A
IBIB or IBi B
IAIB AB
ii O
Sex-Linked traits
most on X
any male that gets an X-linked recessive will express it
red-green colorblindness
Hemophilia- inability to clot blood
1:10 000 males
1: 1 000 000 females
treatment with factor VIII- missing protein responsible
for clotting
Polygenic inheritance- skin color, eye color
Karyotyping
Used to detect missing or abnormal chromosomes
Mitotic cells in metaphase are photographed
pictures of chromosomes cut up and arranged in pairs
according to location of centromere (homologous pairs)
