7.1MENDEL & ONE TRAIT CROSSES

Lesson Objectives Students will be able to:

Explain the major experiments conducted by Gregor Mendel

Define genes, alleles, genotype, phenotype

Demonstrate mastery of one-trait crosses by using punnett squares to complete genetics problems

History: Gregor Mendel 1822-1884 Austrian monk that studied pea

plants Papers were rediscovered in 1900 Called Father of Genetics

Genes & Alleles Genes:

Sequence of nucleotides on chromosome Codes for a protein

Alleles: Variations of a gene (create versions of the

same protein) Example: tall & short Each individual has two alleles for each

gene - diploid One allele from each parent (gametes fused) Homologous chromosomes

Genes & Alleles Alleles can be:

Dominant: always shown Tall: “T”

Recessive: Masked or covered up by dominant allele Short: “t”

Genotype Total of all alleles; genetic make-up

Two alleles Ex: TT, Tt, tt

Genotype types: Homozygous (same two alleles)

Homozygous dominant = TT Homozygous recessive = tt

Heterozygous (different alleles) One of each allele (Tt)

Phenotype Physical appearance of trait Examples:

Tall, short, straight, curly, etc.

Mini-quiz; T = tall; t = short Tall plant

Possible genotypes?

Phenotype?

Short plant Possible genotypes?

Phenotype?

Sexual Reproduction: Individuals are diploid – have two

alleles for each gene Human gametes: egg & sperm Gametes are haploid

Only contain one of each chromosome Have one allele for each gene

In sexual reproduction, it is totally random which allele for each gene is passed onto offspring

One-trait crosses

Single Factor Crosses

Let’s practice! Gametes can only have one allele for

each gene Homozygous dominant parent: TT

Gametes: T or T Homozygous recessive parent: tt

Gametes: t or t Heterozygote parent: Tt

Gametes: T or t

Let’s Practice! Cross: TT x tt

Determine all possible combinations of gametes One from each parent

How? Using Punnett square

Punnett Squares Cross: TT x tt

TT gametes: T or T tt gametes: t or t

F1 Genotype:

F1 Phenotype:

T T

t

t

Tt Tt

Tt Tt

Punnet Squares Cross: Tt x Tt

T = tall; t = short Tt x Tt

Gametes from parent #1: T or t Gametes from parent #2: T or t

Make a punnett square representing this cross and its results

Tt x Tt

TT TtTt tt

T t

T

t

TT

Tt

Tt tt

F1 cross results T = tall; t = short Genotypes: TT, Tt, tt

Genotypic ratio: 1:2:1 Phenotypes: tall, short

Phenotypic ratio: 3: 1

Practice Problem T = tall; t = short Cross a heterozygous tall plant with a

homozygous recessive plant What are the expected genotypes and

phenotypes of the offspring?

7.1 Practice Problems