Mendelian Genetics

Lecture # 4

Chapter 3

9/11/03

1. OVERVIEW OF MENDELIAN GENETICS

Remember why MendelÕs experiments were successful

Observed transmission of simple traits

Used a good Òmodel organismÓ because peas can be both cross-pollinated and self-pollinated

Always used parent plants that were true-breeding or homozygous for a particular trait

Used quantitative analysis of data

Add to your glossary

Gene = single unit of inheritance

Locus = location of a gene on chromosome

Allele = alternative version of a gene or trait

Dominant = allele that predominates over the other allele

Recessive = allele that is masked by the other allele

Phenotype = physical expression of a trait (measurable or observable)

Genotype = actual genetic makeup

Homozygous = two copies of the same allele

Heterozygous = one copy of each allele

Wild-type allele = allele occurs most frequently in nature, product is often functional

Mutant allele = contains modified genetic information

Linked gene = different genes on the same chromosome

Unlinked gene = different genes on different chromosomes

Be familiar with genetic symbols

Capital letter vs small letter for gene names, often named after the mutant or recessive trait, gene alleles are always italicized.

A = dominant allele

a = recessive allele

Diploid genotype will have two letters Ð

Aa = genotype of heterozygote

AA = genotype of dominant homozygote

aa = genotype of recessive homozygote

Haploid genotype (of gamete) will have one letter

Gametes produced by Aa individual will be A or a

Cross vs self symbol Ð

Male vs Female symbol Ð

2. MONOHYBRID CROSS - Defined as a cross between two plants that are heterozygous for a single character trait

MendelÕs method for conducting a monohybrid cross

Start with homozygous true breeding plants = P generation (parental)

Cross them to produce monohybrid = F1 generation (first filial)

Self the F1 offspring to produce the F2 generation (second filial)

Count # of offspring and record phenotypes of F1 and F2 offspring

Monohybrid cross in terms of gametes involved:

If a parent has heterozygous genotype then:

50% of the gametes will have the dominant allele

50% of the gametes will have the recessive allele

F1 monohybrid cross will generate:

2¹ possible male gametes

2¹ possible female gametes

2² possible offspring

See Fig 3.2 in text

Carry analysis one step further to the F3

MendelÕs derived two principles from results of monohybrid crosses:

1. Principle of Dominance Ð One allele may conceal the presence of the second allele

2. Principle of Segregation Ð Two different alleles will segregate away from each other during meiosis.

3. DIHYBRID CROSS - Defined as cross between two plants that are heterozygous for two unlinked character traits

Method of conducting dihybrid cross

Start with homozygous true breeding plants for two different traits = P generation (parental)

Cross them to produce the dihybrid F1 and record the phenotype

Self the F1 offspring to produce the F2 and record the # of offspring and their phenotypes

Dihybrid cross in terms of gametes involved.

Principle of segregation predicts F1 dihybrids should produce 4 different gametic genotypes at equal frequency (25%)

F1 dihybrid cross will generate:

2² possible gametes from male

2² possible gametes from female

2⁴ possible genotypes in offspring

See Fig. 3.2 in text

MendelÕs third principle derived from results of dihybrid crosses:

3. Principle of Independent Assortment Ð Alleles from different genes (located on different chromosomes) assort / segregate independently from one another.

4. METHODS OF PREDICTING SEGREGATION RATIOS

Use Punnett Squares for one or two genes

Use a shortcut for one gene

Use Forked Line Method for two or more

Use Probability Theory to predict probable outcomes for multihybrid crosses

Laws of Probabilities: The Product Law

Product Rule allows you to calculate the outcome of multiple independent events, by multiplying the independent events together

Product Rule = if two events are independent, then the probability that they will occur together is equal to their product

Probability of A and B = P(A) x P(B)

EXAMPLE - Use the product rule to predict outcomes in a trihybrid cross

Trihybrids are the F1 generated by crossing true breeding parents for three unlinked genes and are thus triply heterozygous

Aa Bb Cc X Aa Bb Cc

How many different gametes from a trihybrid individual?

How many different genotypes from a trihybrid cross?

How many different phenotypes from a trihybrid cross?

What is probability of getting a particular gamete?

What is probability of getting a particular phenotype?

What is probability of getting a particular genotype?

ANOTHER EXAMPLE

Aa Bb Cc Dd X Aa Bb Cc Dd

How many different gametes are formed?

How many different genotypes will be produced?

How many different phenotypes will be produced?

Gamete probability:

p (A B C D) =

p (A b C d)=

Genotype probability:

p (aa bb cc dd) =

p (AA Bb cc Dd) =

Phenotype probability:

p (A- bb C- D- ) =

p (aa B- cc D- ) =

Laws of Probabilities: The Additive Law

Sum Rule allows you to calculate the outcome if an event can happen in two or more different ways, by multiplying the independent events first, then adding the events that are specified

Sum Rule Ð if two events are independent, that probability that one OR the other occurs is represented by the following formula

P(A or B) = P(A) + P(B) Ð [P(A) X P(B)]

EXAMPLE using the sum rule:

In an F1 monohybrid cross, what is the probability that the first three F2 offspring will be the same color of flower?

Pp X Pp P = purple flowers

p = white flowers

First, use the product law to calculate the probability of having all purples or all whites:

Probability of three (P-) =

Probability of three (pp) =

Sum the two probabilities to get probability of having all the same color (regardless of whether purple or white):

5. FORMULATING AND TESTING HYPOTHESES IN GENETICS

Test crosses

Used to determine genotypes in individuals that may be heterozygous

Can be used for monohybrids, dihybrids, trihybrids etc.

Conducting a test cross

Start with individual with known traits but of unknown genotype

Cross the unknown genotype with homozygous recessive individual for the traits in question

The ratio of offspring will tell you the original genotype.

Working backwards from offspring numbers to determine genotypes

Sample problem: You self a plant with large, blue flowers and get the phenotypes listed below. Determine the genotype of the parent cross and of the offspring

60 small blue flowers

57 large white flowers

182 large blue flowers

21 small white flowers

Importance of memorizing / recognizing ratios !

Phenotype Genotype

Ratios Ratios

Monohybrid cross 3:1 1:2:1

Dihybrid cross 9:3:3:1 1:2:2:4:1:2:1:2:1

Monohybrid test cross

Dihybrid test cross