Mendel and Genetics: Peas & Probability

Question 1

Genetics at the time of Mendel

Answer 1

1800s - no one knew about DNA or chromosomes
•Practical genetics i.e. artificial selection for domesticated animals and crops, had been going on for 1000’s of years, but no one understood how visible traits were inherited

Question 2

Two prevailing theories:

Answer 2

1. The homunculus theory
   •The sperm-producing parent donates an entire copy of itself to the offspring; the egg only provides nutrients.
   •Problem - many offspring look somewhat like both parents, and some look more like their mom

• 2. The blended theory of inheritance
• The sperm and egg contain “essential essences” that mix together in offspring
• Problem - siblings don’t all look alike

Question 3

Why the pea?

Answer 3

Cross-pollination and selfing

Question 4

Allele

Answer 4

Alternative version of a gene

Question 5

Character (Trait)

Answer 5

Characteristic of an individual that is transmitted from one generation to generation.

Question 6

Cross

Answer 6

Mating between two individuals leading to the fusion of gametes.

Question 7

Diploid (2n)

Answer 7

Eukaryotic cell with two homologous sets of chromosomes.

Question 8

F1 Generation (First Filial Generation)

Answer 8

Progeny of mating of individuals in P (parental) generation

Question 9

F2 Generation (Second Filial Generation)

Answer 9

Progeny of mating (interbreeding) individuals from the F1 generation.

Question 10

Gamete

Answer 10

Mature reproductive cell specialized for sexual fusion;

Haploid cells

Question 11

Gene

Answer 11

Factors;
•Determinant of a characteristic of an organism;
•Gene to DNA to RNA to Protein

Question 12

Locus (loci)

Answer 12

Specific location of gene on a chromosome.

Question 13

True-breeding

Answer 13

When trait being studied remains unchanged from parent to offspring for many generations.

Question 14

Haploid (n)

Answer 14

An individual with 1 set of chromosomes

Question 15

Zygote

Answer 15

Cell produced from fusion of male & female gametes.

Question 16

Dominant

Answer 16

defined by the phenotype that is expressed in an F1 individual (heterozygote)

•Dominant allele often encodes functional protein

Question 17

Recessive

Answer 17

– defined by the phenotype that is not expressed in an F1 individual
Recessive allele often encodes a non-functional protein or lacks expression of a protein

Question 18

Phenotype

Answer 18

observable characteristic of an organism

•Gk “the form that is shown”

Question 19

Genotype

Answer 19

pair of alleles present in an individual

Question 20

Homozygous

Answer 20

two alleles of trait are the same; homozygote = pure line
•Homozygous dominant e.g. Y/Y
•Homozygous recessive e.g. y/y

Question 21

Heterozygous

Answer 21

two alleles of trait are different (Y/y); heterozygote = hybrid

A heterozygote for unexpressed recessive allele is sometimes called a carrier, particularly in humans

Question 22

Hypotheses

Answer 22

1. The differences between alternative traits is due to differences in discrete hereditary determinants or factors (genes)

• 2. The factors exist in pairs, one pair for each characteristic (alleles)
• 3. The members of each pair separate (segregate) equally into the gametes
• 4. Gametes carry only one member of each pair
• 5. Male and female gametes fuse randomly to form the first cell (zygote) of a new progeny

Question 23

Probability

Answer 23

Probability:
•The number of times an event happens ÷ the number of opportunities for an event to happen.

• Product rule:
• The probability of two independent events occurring simultaneously is the product of each of their respective probabilities
• Sum rule:
• The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

Question 24

Mendel’s hypotheses were supported by his data:

Answer 24

1. there are hereditary determinants of a particulate nature (now called genes)
2. each plant has a pair of factors (now called 2 alleles of the same gene) and if different alleles are present one is dominant over the other
3. each allele segregates independently into the gametes and each gamete carries only 1 allele per gene

Question 25

Independent assortment

Answer 25

Mendel designed experiments to determine if two genes segregate independently of one another in dihybrids
•First constructed true breeding lines for both traits, crossed them to produce dihybrid offspring, and examined the F2 for parental or recombinant types (new combinations not present in the parents)
•Dihybrid = double heterozygote F1 from parental pure lines differing in 2 genes
•If 2 or more genes are on different chromosomes, the gene pairs are separated by “;” e.g. A/a;B/b
•If they are on the same chromosome, the alleles on the same chromosome are written adjacent and separated from the other chromosome by a “/” e.g. A B/a b or A b/a B
•If it’s not known if they are on the same chromosome, the gene pairs are separated by a “. “ e.g. A/a.B/b