Heredity and Genetics

PRINCIPLES OF GENETICS

A. OVERVIEW

“Like produces like, but not *exactly* like” REVIEW: Sexual reproduction → variation due to chromosomes

*HEREDITY: transmission of traits from parents to offspring

*GENETICS: science of heredity

B. A History Lesson*Gregor Mendel: 1822-1884

*monk in monastery @ Czechoslovakia

*Vienna- studied science and math- became a teacher

*2 yrs later- returned to monastery; plant experiments

*1866- published data from 8 years of experiments

*1900- finally genetics recognized as major development

*Mendel’s Experiments:

*Studied pea plants (b/c clearly express traits like flower color, seed shape, seed color, and plant height)

*large sample sizes- help ensure actual ratios present in population

*crossed “different” plants → ex. Purple flowers x white flowers

*offspring of parental cross = first filial generation (F1)

*offspring of 2nd cross = 2nd filial generation (F2)

*Mendel produced 929 offspring– 705 were purple (3:1 ratio)

*DOMINANT TRAIT: form of a trait that dominates another trait; (prevents expression of another trait) (ex. Purple flowers)

*appears in F1 generation

*RECESSIVE TRAIT: hidden in F1 generation

*MENDEL’S HYPOTHESIS:

*Mendel’s experiments saw recessive trait disappear in F1

generation but reappear in F2 generation

*ASSUMPTION: for each trait, there must be a pair of factors- 1 comes from egg; 1 from sperm

* “Factors” = GENES: contain information for every trait

*Represented by symbols or letters

*Capitals = dominant

*lowercase = recessive

EX. W= purple, w= white

*ASSUMPTION: 2 genes segregate during gamete formation

*offspring = ratio depending on genes present

EX.

WW x ww parents

Ww Ww Ww Ww F1

Ww x Ww

WW Ww Ww ww F2

RESULTS: 1WW : 2Ww: 1ww

3 purple: 1 white (similar to 705: 929)

*EASIER TO SEE WITH PUNNETT SQUARES:

*Chart used to determine possible combinations of genes among offspring

Gametes of 1 parent

Gam

etes of 1 parent

EX: Ww x Ww

WW WwWw ww

W w

W

w

*TEST OF SEGREGATION:

3:1 ratio 1st time

but what if Ww x ww???

MAKE YOUR PUNNETT SQUARE:

ww

Ww

Ww

ww

*****50% white:

50% purple!!!

W w

w

w

GENETICS TERMINOLOGY

*ALLELE: either of 2 alternative forms of gene for particular trait

ex. W or w*GENOTYPE: combo. of alleles for certain trait

ex. WW, Ww, or ww*PHENOTYPE: appearance of trait

ex. “purple” or “white”

*HOMOZYGOUS DOMINANT: 2 dominant alleles (WW)

*HOMOZYGOUS RECESSIVE: 2 recessive alleles (ww)

*HOMOZYGOUS: 2 of same alleles for given trait

ex. RR, rr (deals w/ seed shape) or WW or ww

*HETEROZYGOUS: 2 different alleles for a trait

ex. Rr or Ww

*are used to help determine the possible genotypes of parents and offspring

*are used to analyze the chromosomes present in a person

TIME TO PRACTICE

PUNNETT SQUARES!!