Dihybrid Inheritance

Learning intention: To develop an understanding of Inheritance pattern - Dihybrid

Success criteria: Discuss how dihybrid inheritance causes variation in gene pools

Dihybrid Inheritance

Dihybrid inheritance is the study of the inheritance of two characteristics. At level 2, it will always involve traits demonstrating complete dominance.

Let's revisit Mendel and his pea plants to understand dihybrid inheritance!

Mendel’s Experiment

Mendel chose to cross a pea plant pair with two pairs of contrasting traits. These traits are seed color and seed shape. One of the plants is homozygous for the dominant traits of round seed shape (RR) and yellow seed color (YY). Thus, together the genotype is expressed as (RRYY). The other plant is homozygous for the recessive traits of wrinkled seed shape (rr) and green seed color (yy). Together the genotype is expressed as (rryy).

F1 Generation - A true-breeding pea plant with a round seed shape and yellow seed colour (RRYY) is cross-pollinated with another true-breeding plant with a wrinkled seed shape and green seed colour (rryy). The resulting F1 generation is all found to be heterozygous for round seed shape and yellow seed colour (RrYy). All F1 offspring were found to be phenotypically identical, producing yellow seed colour and round seed shape.

F2 Generation - Mendel continued with his experiment with the self-pollination of F1 progeny plants. To his surprise, the plants exhibited a 9:3:3:1 phenotypic ratio of seed shape and seed colour. Nine out of the sixteen plants were found to exhibit round, yellow seeds. Three of them exhibited round green seeds. Another three gave wrinkled, yellow seeds, and the remaining one plant gave wrinkled green seeds. Thus, F2 generation exhibited four different phenotypes and nine different genotypes.

Genotypic and Phenotypic Ratios

Genotypes determine the phenotype in an organism. Thus, a plant exhibits a specific phenotype based on whether its alleles are dominant or recessive. One dominant allele leads to a dominant phenotype being expressed. The other way for a recessive phenotype to appear is for a genotype to possess two recessive alleles. Both homozygous and heterozygous dominant genotypes are expressed as dominant.

In the experiment performed by Mendel, yellow (Y) and round (R) are dominant alleles, and green (y) and wrinkled (r) are recessive. The possible phenotypes and their genotypes are given below:

How to Make a Dihybrid Cross Punnett Square

The above result is represented using a 4 x 4 Punnett square. All the four possible combinations of gametes for yellow seed color and round seed shape pea plant are placed from top to bottom of the first column. For green seed color and wrinkled seed shape, pea plant in the top row from left to right. The Punnett square is given below:

What is a Dihybrid Test Cross

It is a cross that helps to explore the genotype of an organism based on the offspring ratio. When an organism’s genotype expressing a dominant trait is not known to be homozygous or heterozygous, we need to perform a test cross. A dihybrid test cross is done involving two pairs of contrasting characters.

In a test cross, an individual with an unknown genotype is crossed with a homozygous recessive individual. The unknown genotype can be obtained by analyzing the phenotypes in the offspring. The result of a dihybrid test cross-ratio is represented using a Punnett square. If the unknown genotype is heterozygous, a test cross with a homozygous recessive individual will result in a 1:1:1:1 ratio of the offspring’s phenotypes.

Example

Here, one of the heterozygous parents for seed color and seed shape (RrYy) is crossed with a homozygous plant for both the traits (rryy). The test cross produces four possible genetic combinations RrYy, Rryy, rrYy, and rryy in a ratio of 1:1:1:1.

Thus, a pea plant’s genotype producing dominant round and yellow seeds can be determined when test crossed with a wrinkled green plant having recessive alleles for both the traits.

Copied from: https://www.sciencefacts.net/dihybrid-cross.html

Finding the Gamete Genotypes

To find the potential genotypes of the gametes produced by each parent in Dihybrid Inheritance, we can use the F.O.I.L technique.

First letter from each gene

Outside letter from each gene

Inside letter from each gene

Last letter from each gene

For example:

A parent pea plant is heterozygous for both round and yellow traits. It's genotype is RrYy. The potential gametes are:

First: RrYy, Gamete genotype is RY

Outside: RrYy, Gamete genotype is Ry

Inside: RrYy, Gamete genotype is rY

Last: RrYy, Gamete genotype is ry

ACTIVITY: Now, practice F.O.I.L yourself in the document below!

Dihybrid Crosses FOIL

ACTIVITY: Watch the video and complete the worksheet!

Biozone Activities

E78 - Dihybrid Cross, pp. 108 - 109

Gene Linkage

Gene linkage means that two genes are likely to be inherited together because they lie closely together on a chromosome.

When two genes are close together on the same chromosome, they do not assort independently and are said to be linked. Whereas genes located on different chromosomes assort independently and have a recombination frequency of 50%, linked genes have a recombination frequency that is less than 50%. This means that they are not likely to separate during crossing over.

Will this increase or decrease genetic variation?

Crossing over and recombination does not happen in every cell during meiosis. This means that genes that are linked (i.e. have a recombination frequency that is less than 50% of the time) are likely to be inherited together rather than ‘randomly’ due to independent assortment and segregation. This results in the inheritance of these two genes deviating from the Mendelian pattern of 9:3:3:1.

Linkage and crossing over do not affect inheritance if both parents are homozygous i.e., HHGG or hhgg in the example below. However they do affect the inheritance pattern of heterozygous genotypes.

Gametes for unlinked and linked genes:

Linked Genes Resource Gallery

Video format:

Text and Visual format:

Ben Himme's Pathwayz.

Biozone Activities

E79 - Inheritance of Linked Genes, pp. 110 - 111

E80 - Recombination and Dihybrid Inheritance, pp. 112 -113

E81 - Problems Involving Dihybrid Inheritance, pg. 114

Page updated

Report abuse