Chapter 10Patterns of Inheritance

10.1 Genetics

trait: a variation of a particular characterEx yellow or red flowers

gene: a unit of inherited information

Blending Hypothesis1800

Explains how offspring inherit traits from both parents Ex. yellow flower + red flower = orange flower

Discarded because it could not explain traits disappearing and reappearing in another generation

Gregor Mendel19th century Austrian

Monk

First to apply an experimental approach to the question of inheritance

genetics: the study of heredity Used pea plant

breeding

Particulate HypothesisParents pass on

to their offspring separate and distinct factors (genes) that are responsible for inherited traitsHeritable factors

retain their identity through generations

Mendel’s Pea Plants

Mendel’s Experiment

1. Identify pea plants that were true-breeding When self-fertilized, only produces identical offspring

2. To insure self-fertilization, Mendel tied cloth bags around each plant’s flowers to prevent cross pollination

3. Mendel crossed true-breeding plants with two distinct traits (color and roundness)

cross-fertilization: sperm from the pollen of one flower fertilizes the eggs in the flower of a different plant

4. The fertilized eggs developed into embryos within seeds (peas) that were planted

5. These then grew to produce flowers Purple crossed with white flowers

10.2 Mendel’s Principle of Segregation

hybrids: the offspring of two different true-breeding varieties

P generation= parental

F1 generation = the hybrid offspring

F2 generation = offspring when F1 self-fertilize each other

Monohybrid Cross monohybrid cross: a

pairing in which parent plants differ in only one character Mendel found that F1

produced all purple flowers but F2 3/4 purple and 1/4 white

He repeated this for other traits (seed color, shape, pod shape & color, flower position, and stem length)Same result

Mendel’s 4 Hypotheses1. There are alternate forms of genes

alleles: the alternate forms of genes Ex. Purple and white

2. For each inherited character, an organism has 2 alleles for the gene controlling that character, one from each parent

homozygous: 2 alleles are the same for that character

heterozygous: the 2 alleles are different

3. dominant: only one allele appears to affect the trait

recessive: the allele that does not appear to affect the traitPp; P = dominant, p = recessive

4. Principle of SegregationTwo alleles for a character separate during

formation of gametes (meiosis) so that each gamete carries only one allele for each character

Punnett SquareDiagram that shows all possible

outcomes of a genetic cross

Phenotype and Genotype phenotype: an observable trait

Hair color, height, tongue rollingThe dominant trait shows upBb and BB = dominant; brown bb = recessive; blond

genotype: the genetic makeup (combination of alleles)

1/2 Bb; 1/2 bb = 2:2

Test Crosstest cross: breeds

an individual of unknown genotype but dominant phenotype with a homozygous recessive individual

Principle of Independent Assortment

During gamete formation in an F2 cross, a particular allele for one character can be paired with either allele of another

Dihybrid Cross

dihybrid cross: crossing organisms differing in 2 characters

10.3 Variations of Inheritance Patterns

Intermediate Inheritance(Incomplete Dominance)

intermediate inheritance: the heterozygotes have a phenotype that is intermediate between the two homozygotesEx. Red + white

flowers = pink, red & white

Codominance codominance: a heterozygote

expresses both traits

Ex. Blood Type A, B, AB, O A & B refer to 2 carbohydrates that are

found on the surface of red blood cells

O = ii = neither carbohydrates Universal donor

A= IAIA or IAi = A carb (A)

B= IBIB or IBi = B carb

AB= IAIA or IAi = both A & B Universal receiver

Polygenic Inheritance polygenic inheritance:

two or more genes affect a single character Ex. Eye color, skin,

height

Eye color = tone of pigment , amount, and position

pleiotropy: a single gene affects more than one trait Ex sickle cell anemia

Importance in the Environment

Temperature can have an effect on animal coloringEx siamese cats,

rabbits

Height can be affected by nutrition and exerciseEx. Dancers

Nature vs Nurture

10.4 Meiosis Explains Mendel’s Principles

Chromosome Theory of InheritanceStates that genes

are located on chromosomes and the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns

Genes gene locus: the

alleles of a gene reside at the same location on homologous chromosomesThe homologous

chromosomes may bear the same or different alleles

Genetic Linkage genetic linkage: the

tendency for the alleles on one chromosome to be inherited together The closer the genes are on

a chromosome, the greater the genetic linkage and crossing over will not separate them

The probability of genetic linkage is measured in centiMorgans (cM)Represents 1% of the

probability of crossing over1 cM apart = 1% chance of

separation20 cM = 20% chance

10.5 Sex-Linked Genes

sex-linked gene: any gene located on a sex chromosome Most are found on the X

chromosome

Thomas Hunt Morgan Discovered sex-linked genes

while studying fruit flies Normal fruit flies have red eyes

which is carried on the X, but sometimes males have white (recessive)

Sex-linked Traits in Humans

ColorblindnessX-linked recessiveLack of red

receptors in the retina8% males0.4% females