LAB LECTURE 2

Mendel’s 1st law of Segregation

States that homologous chromosomes separate doing meiosis (anaphase I), resulting in each gamete receiving only 1 allele for a particular trait

Monohybrid crosses reveal that

Each diploid organism possesses 2 alleles for any gene

2 alleles separate when

Gametes are formed; 1 allele goes into each gamete (on 1 chromosome)

Alleles separate with

Equal probability into gametes

Mendel’s 2nd law of Independent Assortment

Inheritance of one gene during prophase I does NOT affect the inheritance of another gene; copies of different genes assort independently

Chromosomes segregate

Independently in meiosis during formation of gametes

Alleles on different chromosomes do not

Travel together! (separately)

Punnett squares ensure

All possible combinations of gametes when calculating expected genotype frequencies

Two traits of corn kernel controlled by genes

Color, texture

Each kernel is

An individual offspring; kernel = seed

Corn is classified as a…

Monoecious plant

Monoecious plants

Produce both male and female gametes on the same plant

Two methods of corn reproduction

Either undergo self-fertilization or cross-pollination (with another plant)

Chromosomes in corn

Has 10 pairs of chromosomes

Corn haploid + diploid numbers

Gametes are haploid (n) = 10

Plants are diploid (2n) = 20

3 basic components of kernel anatomy (external → internal)

• Pericarp

• Aleurone

• Endosperm

Pericarp

2n; usually transparent, outermost layer of kernel, is the maternal origin & remnant of the pistil

Aleurone

3n; middle layer of kernel and its coloration will mask that of endosperm (so if colorless, then color of kernel is that of endosperm)

Endosperm

3n; inner layer, product of double fertilization and provided nutrition for the embryo, also pigmented

Aleurone color is due to combination of:

1. Ability to make anthocyanin pigment

2. Anthocyanin pigment color

Ability to make anthocyanine pigment is regulated by

A, C, & R genes

Anthocyanin pigment color is determined by

An independent locus for the aleurone gene Pr

Purple pigmentation is caused by

Pr/PR & Pr/pr genes

Red pigmentation is caused by

pr/pr

Rule for aleurone genes A,C,R

All 3 genes must concurrently have at least 1 dominant allele expressed to produce anthocyanin

• If any one of the 3 genes are homozygous recessive, no pigment will result

Pigment produced? Aa, CC, rr

No (homorec)

Pigment produced? AA, Cc, RR

Yes

Epistasis

A type of gene interaction in which 1 gene alters the phenotypic effects of another gene that is independently inherited

Epistatic gene

The gene that masks the effect of another gene at a different locus

Hypostatic gene

The gene affected by the epistatic gene

Dominant epistasis

When the dominant allele of one gene masks the expression of all alleles of another gene

Recessive epistasis

When the recessive allele of one gene masks the effects of either allele of the second gene

Cl

A dominant epistatic gene that is an allele of the C locus

Purpose of Cl

Inhibits color production in the aleurone regardless of the genotypes for the A, R, or Pr loci

Relation of genes A, R, and Pr to Cl

Are hypostatic to Cl

Gene Y determines

Production of carotenoid pigment, which results in a yellow color

Y pairing possibilities

• YY/Yy allow production of carotenoid, so yellow

  • yy does NOT allow production, so white

Corn texture and its determinant

Usually starchy; determined by endosperm

Type of starch in corn

Amylose

Sugary gene in corn & its purpose

Su; separates sweet corn from field corn

Su pairings and expressions

SuSu/Susu = starchy/smooth kernels that are opaque when dried

su/su - sugary/wrinkled kernels that are translucent when dried

All (P) corn varieties are

Homozygous for all the genotypes

Chi square test

Statistical test used to determine how well the observed phenotypic ratios match the expected ones, given the hypothesized pattern of inheritance

Results of chi-square

• If results indicated that the difference could be due to chance, then hypothesized pattern of inheritance is supported

• If results indicate that difference is likely not due to chance, the difference is significant and pattern of inheritance is rejected

Null hypothesis

H₀, states that variation between O and R is due to chance

Alternative hypothesis

H_A, states that variation between O and E is not due to chance alone, therefore difference is statistically significant

Rule about statistics and our hypothesis

Help us support, not prove, whether a hypothesis is acceptable (not necessarily correct) within a certain measure of error

Chi square formula

P value and its significance

0.05; subtract 1 from # of groups for degree of freedom, then find corresponding value, compare to result

• < 0.05 value: is statistically sig, reject null

• > 0.05 value: is not statistically sig, accept null