Cell Division Part II: Inheritance

Selective breeding

Selective breeding- choosing parents with desirable traits (physical, behavioral, mental...) and have them produce offspring.

Gregor Mendel

worked with garden peas to explain inheritance:

“Father of genetics”

Explained “true-breeding”- All offspring resulting from parental crosses have the same phenotype.

Alleles

Alternate forms of genes.

Mendels Experiments

P generation- Crossed 2 true breeding plants

(purple & white flower)

F1 generation- all purples. Allow these to self fertilize

F2 generation- 75% purple: 25% white

mixing of 1 trait: MONOHYBRID CROSS

Dominant vs Recessive:

If a dominant gene is present it over-rules

the recessive gene, and the dominant trait

will be expressed- no intermediate, or mix.

(complete dominance)

Mendels laws of heredity

1. Segregation

   Inherited factors are controlled by paired genes. During sex/cross-pollination each parent contributes one of its genes or alleles.

   Not all copies of a factor are identical.

   homozygous - same alleles (ex. RR or rr)

   heterozygous - different alleles (ex. Rr) One factor masks another (dominance).

   Because a factor is masked does not mean it is not still there. Dominant allele: upper case R- round seed

   Recessive allele: lower case r-wrinkled seed

   Gene pairs separate during formation of sex cells

   (meiosis)- law of segregation.

   Haploid sex cells from diploid parent cell.

Maternal and Paternal homologue

In a pair of homologous chromosomes, one is inherited from the mother and one from the father.

Genetic locus

A genetic locus is the location of a particular gene on a chromosome. At each locus, an individual has 2 alleles, one on each homologous chromosome.

Monohybrid cross

Monohybrid = looking at One Trait

Hybrid P-gen – Ww X Ww (heterozygous)

Two alleles per parental trait, gametes are

haploid.

Punnett Square with 4 boxes = 4 possible offspring combinations.

Monohybrid Cross-autosomal recessive

Not affected by sex and the individual must inherit both recessive alleles to express the trait

2nd law of heredity: Independent assortment

Genes assort independently (no influence) of each other during gamete formation.

Dihybrid Cross

Dihybrid – 2 separate traits

(ex seed colour & shape). Gametes per parent = 4

Punnett Square – 16 boxes

Genotype ratio

1:2:1:2:4:2:1:2:1

Phenotype ratio

9:3:3:1

Genes of shape and colour are inherited

independently

– Law of independent assortment.

Incomplete dominance

Blend of two equally dominant traits (both

expressed at the same time)

F2 Genotype frequency 1:2:1

F2 Phenotype frequency 1:2:1

Examples: Flowers, Sickle-cell anemia

Sickle cell anemia

Homozygous normal- HbA HbA

(normal) -good O2 transport

-susceptible to malaria

Heterozygous- HbA HbS (normal & sickle cell) -some malaria resistance

-trouble w/ O2 transport

Homozygous disordered- HbS HbS

(Sickle cell disease) -malaria resistant

-bad O2 transport

Co dominance

Both alleles are fully expressed. (no blending)

Thomas Hunt Morgan

American geneticist (1866-

1945) studied mutations and recessiveness (white eye) in fruit flies to discover

sex-linked traits. Morgan concluded that

because the X and Y (sex) chromosomes are not homologous, they must contain different genes. (non-linked genes)

Eye colour is located on the X chromosome

Traits on the sex chromosomes are sex- linked traits.

Sex linked inheritance

Sex-linkage – genes on sex chromosomes (X or Y, but especially X.

Y-chromosome shorter – some genes from X missing.

X-linked traits more common in men

Men get X-chromosome from mom

Examples: baldness, hemophilia, Red-green colorblindness

Color blindness

Determined by a recessive gene located on the X chromosome (ex. Xc)

Males: more common (only one X)

Females: need recessive gene on both X’s to be colour blind (ex. XcXc)

Barr Body

Tightly condensed, inactive X chromosome in females.

Polygenetic Inheritance

Continuous/Polygenic traits:

Groups of genes that contribute to the same trait.

traits for which phenotypes vary gradually from one extreme to another (along a scale/continuum)

traits are controlled by more than one gene. (Ex. human height, skin & eye

colour, corn ear length).

# of dominant genes produces

different results.

Genetic interactions

Epistatic: genes that interfere with

expression of other genes (ex. Dog coat colour)

Complementary: 2 different genes

interact to produce a phenotype that neither is able to produce by itself.

Pleiotropic: affect many different

characteristics

Crossing over in inheritance

exchanging genes with homologous

pair, occurs more frequently between genes further apart on a chromosome, than those closer together.