MENDELIAN GENETICS Punnet Squares and Pea Plants

Introduction

Mendelian laws of inheritance are statements about the way certain characteristics are transmitted from one generation to another in an organism.

Mendel’s Pea Plants

For his experiments, Mendel used ordinary pea plants.

He examined the following traits: color of a plant's flowers location on the plant shape and color of pea

pods shape and color of

seeds length of plant stems.

Generations

He found that certain traits disappeared in the first generation and reappeared in the second generation.

Prior to his research, it was a commonly held belief that heredity was a blending of the traits of the two parents

For example, combining the colors blue and red will give you the color purple.

Something to keep in mind is that in Mendel’s time, cell biology was very primitive and the idea of a nucleus with chromosomes was unknown.

Genes and Alleles

A Gene is a molecular unit of hereditary of a living organism.

The arrangement and number of genes creates the specific organism.

In Mendel's time, he referred to genes as ‘unit factors’. Genes are created when 2 alleles come together. One

allele from the female and one allele from the male. The combination of these alleles will create a specific

outcome for that organism. Mendel called alleles ‘2 factors that control each trait’.

Combinations of Genes

Mendel stated that each parent passed on their genes in an unblended or pure state, however the trait expressed depends on the combination of alleles that were passed on.

Combinations of Genes

Dominant alleles mask the expression of recessive alleles. Only one copy of the allele is needed for the trait to be expressed.

Recessive alleles are only expressed in the homozygous (both alleles the same) state. Must have two copies for the gene to be expressed.

Gene Combination Types - Homozygous

Homozygous – alleles for a trait are the same. Can be expressed in 2 different ways. Homozygous dominant – Both alleles of a gene are of the

dominant nature. Represented by 2 capital letters of the same letter. (e.g. TT )

Homozygous recessive – Both alleles if a gene are of the recessive nature. Represented by 2 lower case letters of the same letter. (e.g. tt)

It is not enough to say a gene is homozygous. This simply implies both alleles are the same but does not indicate whether they are of a dominant or recessive nature.

Mendel referred to these as ‘true breeding’ or ‘pure bred’ traits.

Gene Combination Types - Heterozygous

Heterozygous – Alleles for a gene are different. Same letter is used, but one is a capital and one is a lower case. (e.g. Tt)

The dominant trait is expressed as it masks the expression of the recessive trait, even though the recessive trait is present.

Mendel referred to heterozygous as hybrid.

Genotype VS Phenotype

Genotype – the genetic make up of an organism 3 types – Homozygous dominant, Homozygous recessive

and Heterozygous. Represented by letters. Phenotype – physical trait that an organism develops. Dominant or recessive trait. Example: A Homozygous dominant brown eyed (BB)

person and a Homozygous recessive blue eyed person (bb) mate.

Generations

When analyzing the genetic history of organisms, they are divided into generational patterns.

This allows a geneticist to trace backwards direct traits.

This also allows for the understanding of dominant versus recessive traits in unknown organisms, as it illustrates the dominant pattern throughout breeding.

Genetic Crosses – Generations

Parental Cross – cross between the parents. Sometimes called ‘P cross’.

F1 Generation – first

generation, result of P cross

F2 Generation – second

generation, result of 2 F1 generation offspring crossing.

P

MENDEL’S LAWS

LAW OF SEGREGATION This law states that, for

any particular trait, its two alleles (alternate versions of the same gene) separate so that only one is passed to the offspring.

Which of the two alleles is inherited is left to chance, thus explaining variations within siblings.

LAW OF INDEPENDENT ASSORTMENT This law states that

traits are expressed independently of each other. For example, inheritance and expression of the genes for blue eyes does not directly affect inheritance and expression of the gene for hair color.

Punnet Square

A diagram showing the results of any cross of genes in an organism.

Illustrates the separation of alleles and their possible outcomes.

Always only 2 members of a species are crossed at a time.

Multiple genes can be crossed at once.

Example of a Punnet Square

Parent 1

Parent 2

Can you name the genotypes of each of the parents?

Monohybrid Cross

A genetic cross in which only one pair of contrasting traits is studied.

e.g. Tt x TT (only the trait height is being studied)

Example 1 – Monohybrid Cross Question: In humans, long eyelashes are

dominant and short eyelashes are recessive. A heterozygous woman with long eyelashes and a homozygous dominant man with long eyelashes have children.

a) List the probable genotypes

of the parents b) List the probable genotypes

and phenotypes of the children

Procedure: 1) Make a key and assign a

letter. 2) State the cross. 3) Make the square and fill in

parental genotypes. 4) State the results in

genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Example 2 – Monohybrid Cross – Let’s get tricky! Question: In humans, long eyelashes are

dominant and short eyelashes are recessive. A woman with long eyelashes and a man with long eyelashes have four children. One child has short eyelashes, the others have long eyelashes.

a) List the probable genotypes

of the parents b) List the probable genotypes

and phenotypes of the children

Procedure: 1) Make a key and assign a

letter. 2) State the cross. 3) Make the square and fill in

parental genotypes. 4) State the results in

genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Example 3 – Tricky again!

Peas may have yellow or green

seeds. A cross between a green seed plant and a yellow seed plant produced all yellow seeds. Crosses between two of the F1 generation produced 58 yellow and 19 green.

a) Identify the genotypes of the P

(parent) generation b) What would the phenotype ratio

of yellow seeds to green seeds be if one plant from the F1 was crossed with the yellow seed plant from the P generation?

Procedure: 1) Make a key and assign

a letter. 2) State the cross. 3) Make the square and fill

in parental genotypes. 4) State the results in

genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Test Cross

The individual of an unknown genotype is mated with an individual showing the contrasting recessive trait.

This will help to determine if the unknown genotype is first dominant or recessive. If dominant, if it is homozygous dominant or heterozygous.

The genotype of the recessive individual is always homozygous.

Example 1

A test cross is performed on a tall plant of unknown genotype. The offspring that result are ½ short and ½ tall. What is the genotype of the tall parent if tall is dominant to short?

Procedure: 1) Make a key and

assign a letter. 2) State the cross. 3) Make the square and

fill in parental genotypes.

4) State the results in genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Example 2

A test cross is performed on a tall plant of unknown genotype. The offspring that result are 100 %tall. What is the genotype of the tall parent if tall is dominant to short?

Procedure: 1) Make a key and

assign a letter. 2) State the cross. 3) Make the square and

fill in parental genotypes.

4) State the results in genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Dihybrid Cross A genetic cross in which

two pairs of contrasting traits are studied

Example: Parent 1:YYRR crossed with Parent 2:yyrr Y- dominant yellow colour y- recessive green colour R – dominant round seed r – recessive wrinkled seed

State the phenotypes of each parent: Parent 1: Parent 2:

Phenotype: 100% yellow and round Genotype: 100% hybrid for both traits

Dihybrid Cross

Example: Parent 1: YyRr crossed with Parent 2: YyRr

Y- dominant yellow colour y- recessive green colour R – dominant round seed r – recessive wrinkled seed

State the phenotypes of each parent: Parent 1: Parent 2:

Phenotype: 9 yellow-round 3 yellow-wrinkled 3 green-round 1 green-wrinkled

Dihybrid Cross Question 1

Example 1: What genotypes result if

one parent is homozygous recessive for attached earlobes and homozygous dominant for widow’s peak hairline, and the other parent is homozygous dominant for unattached earlobes and homozygous recessive for straight hairline?

Procedure: 1) Make a key and assign

a letter for each trait. 2) State the cross. Use the

1,2,3,4 method. 3) Make the square and

fill in parental genotypes. 4) State the results in

genotypic and phenotypic forms using fractions or percentages.

5) ADQ!

Dihybrid Cross Question 2

Try this on your own: Example 2: Widow’s peak is dominant

to straight hairline and short fingers are dominant to long fingers. If two parents who are heterozygous for both traits are crossed, what are the resulting genotypic and phenotypic ratios?

Procedure: 1) Make a key and assign

a letter for each trait. 2) State the cross. Use the

1,2,3,4 method. 3) Make the square and

fill in parental genotypes. 4) State the results in

genotypic and phenotypic forms using fractions or percentages.

5) ADQ!