MENDELIAN

GENETICS

Gregor Mendel

Probability & Punnet Squares

Mendelian Principles

Vocabulary

Dominant ≡ allele which masks the expression of another; represented by capital letters. (B, T)

Recessive ≡ allele which is expressed only if both parents contribute it; represented by small letters.(b, t)

Incomplete dominance ≡ phenotype of the trait isn’t a total masking of trait. Example is red tints in the hair; represented by capital letters and subscripts. (B1, B2)

Vocabulary

Polygenic ≡ several genes contribute to the overall phenotype; example is skin color.

Sex–linked ≡ commonly applied to genes on the X chromosome, the more current term is X-linked; genes on the Y chromosome are holandric genes.

Epistasis ≡ one gene masking the effects of another; and example is hair color to red color tints.

Vocabulary

Phenotype ≡ the physical characteristics from the expression of the genes.

Genotypes ≡ the actual genes that an individual has inherited.

Haploid ≡ having half of the chromosome pairs. (23 single chromosomes for humans)

Diploid ≡ having the complete pairs of chromosomes (23 pairs of chromosomes for humans)

Vocabulary

Homozygous ≡ both genes for a particular trait are the same. (BB or bb)

Heterozygous ≡ both genes for a particular trait are opposite. (Bb)

Gametes ≡ sex cells, the sperm or egg.

Genetics Why do people, even closely related people, look slightly different from each other?

The reason for these differences in their phenotype is the different combination of genes possessed by each individual

Genetics

Each normal human being has 46 chromosomes, diploid, in each body cell.

In forming the gametes one chromosome from each pair will be given, so the gametes are haploid.

Genetics

In this way, you contribute half of the genetic information for the child – your partner will contribute the other half.

In reality there are thousands of different gene pairs, and so there are millions of possible gene combinations.

Gregor Mendel He was an Austrian monk who was

born in 1822.

He is known as the “Father of Genetics”.

He carried out his work on inheritance with ordinary garden peas.

He spent years making sure the pea plants were true breeding.

True Breeding ≡ if allowed to self pollinate, they

would produce offspring identical to themselves.

Pea Plant Traits

Genes & Dominance

Fertilization ≡ A process during sexual

reproduction when the male and female

reproductive cells join.

Trait ≡ a specific inherited characteristic

(Gene)

Genes & Dominance

F1 ≡ “First filial” or

first generation.

Hybrid ≡ a cross

between two true-

breeding parents.

Mendelian Genetics

Mendel made 2 conclusions from his first experiments:

First, biological inheritance is determined by factors that are passed from one generation to the next.

Today we call those factors – Genes.

Second, this conclusion is called “The Principle of Dominance”.

PRINCIPLE OF

DOMINANCE

Some alleles are dominant and others are recessive.

Dominant & Recessive

Allele ≡ a form of a particular trait / gene.

Dominant ≡ an organism with a dominant allele for a particular form of a trait will always exhibit that form of the trait… it will be ‘expressed’.

Recessive ≡ An organism with a recessive allele for a particular form of a trait will exhibit that form ONLY when the dominant allele is not present.

Segregation During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene.

Probability & Punnett Squares

Probability ≡ the likelihood that a particular

event will occur is called probability.

Flipping a coin:

The probability that a single coin flip will

come up heads is 1 chance in 2.

This is ½ or 50%.

The Probability of Coin Tosses

# of Tosses

# landed on Heads

% landed on Heads

(# of heads ÷ # of Tosses) x 100

1

5

10

100

Punnett Squares

Punnett squares can be used to predict and compare the genetic variations that will result from a cross.

Cross ≡ When two organisms sexually

reproduce offspring.

Punnett Squares

Menellian Crosses

(1) BB x bb (2) Bb x Bb

Di-Hybrid Cross (1-3, 1-4, 2-3, 2-4)

(1) RRYY x rryy

Di-Hybrid Cross (1-3, 1-4, 2-3, 2-4)

(2) RrYy x RrYy

Principle of Independent Assortment

Genes for different traits can segregate independently during the formation of gametes.

Independent assortment helps account for many genetic variations observed in plants, animals, and other organisms.

Incomplete Dominance vs.

Codominance

Incomplete Dominance ≡ where neither allele is truly dominant and there is a blending of the traits.

Codominance ≡ Both alleles contribute to the phenotype because neighter allele is dominant. However, unlike the blending you have in Incomplete Dominance, in Codominance, you see both traits showing up separately in the phenotype.

Incomplete Dominance - Codominance

Codominance

Multiple Alleles

Multiple Alleles ≡

When genes have

more than two alleles

that exist in a

population.

EX: coat color in

rabbits, blood type

Polygenic Traits Polygenic Traits ≡ Traits controlled by two or more

genes. These traits are produced by the interaction of

several genes.

EX: skin color in humans, height