animal, plant & soil science lesson b2-4 heritability of traits

Animal, Plant & Soil Science

Lesson B2-4

Heritability of Traits

Interest Approach

Have students count the number of individuals in the class who have brown eyes, blue eyes, and green eyes. Then ask about the eye color of each student’s parents. Discuss the relationship between the parents’ eye colors and the student’s eye color.

Objectives

Explain how to estimate the heritability of certain traits.

Predict the genotypes and phenotypes from monohybrid and dihybrid crosses by using the Punnett square.

Describe sex determination, linkage, crossover, and mutation.

Terms adenine alleles chromosome cytosine DNA dominance double helix eukaryotic gametes

gene expression genes genetic code genetics genome guanine heredity incomplete

dominance

Terms alleles codominance crossover dihybrid cross dominant genotype heritability heritability

estimate

heterozygous homozygous incomplete

dominance linkage monohybrid

cross mutation

Terms

phenotype probability Punnett square qualitative traits quantitative traits recessive sex

chromosomes

How do we estimate the heritability of certain traits?

Estimating is based on probability. Probability is the likelihood, or chance,

that a trait will occur. Mating animals with particular traits does

not guarantee that the traits will be expressed in the offspring.


Heritability is the proportion of the total variation (genetic and environmental) that is due to additive gene effects.

A heritability estimate expresses the likelihood of a trait being passed on from parent to offspring.

If a trait has a high heritability, the offspring are more likely to express that same trait.


A. The genes contained in an animal control the traits of that animal.

Some traits are controlled by only one pair of genes, whereas others require several pairs.

1. Qualitative traits are traits controlled by only a single pair of genes and cannot be altered by the environment. Their phenotype is either one thing or the other. These traits most easily show how genes are

inherited. An example is coat color.


2. Quantitative traits are traits controlled by several pairs of genes. These traits are expressed across a range. These traits can also be altered by

environment. Examples include rate of gain, growth rate,

backfat depth, and so on.


3. Not all traits contained within an organism are expressed. Dominant traits cover up, or mask, the

alleles for recessive traits. In some organisms, there are cases of

codominance of traits in which both dominant and recessive genes are expressed.

Incomplete dominance occurs when a blending of the allele pair is expressed.


4. The Punnett square is a technique for predicting genotype. It considers the

dominant and recessive genes of the male and female parents for one trait.

How do we predict the genotypes and phenotypes from monohybrid and dihybrid crosses by using the Punnett square?

Resulting offspring of reproduction have both genotype and phenotype heredity.

A. Genotype is the actual genetic code. It controls physical and performance

traits. The genotype of an organism cannot

be changed by environmental factors.


B. Phenotype is the organism’s physical or outward appearance. This is the part of the

genotype the organism expresses or shows.

In some instances, phenotype may be altered by the organism’s environment.


C. A homozygous organism is one having similar alleles, or genes, on the DNA molecule for a particular trait, while a heterozygous organism is one having different alleles for a particular trait.

D. The Punnett square is a useful tool to help determine both the genotype and phenotype from animal crosses.


1. A monohybrid cross takes into account a single trait and is represented by a single set of letters. The dominant gene is represented by a capital

letter, whereas the recessive gene is represented by a lowercase letter.

In an example for eye color, brown eye color is dominant over blue, and the trait would be represented “Bb”.

Using a Punnett square, a monohybrid heterozygous cross would have four squares.


2. A dihybrid cross takes into account two traits and would be represented by four letters instead of two. A dihybrid cross showing both eye color and

hair color may be represented as “BbHh”. A typical dihybrid heterozygous cross would

use 16 squares in the Punnett square, making it slightly more difficult.


3. A Punnett square is created by drawing a four-square (two by two) or a 16-square (four by four) box. Each parent is assigned a side of the square, and

all possible offerings from that parent are written along the side, with one offering per square.

4. Each parent is allowed to give one gene per trait. For instance, in the earlier example of eye color, a

parent with the genotype Bb can either give “B” or “b” but not both at the same time.

By combining with the corresponding gene from the other parent, the offspring is assigned an eye color.


5. To determine the genotype and phenotype of the offspring, the parent’s offerings are combined and written in the corresponding inner squares. It is important when writing the

letters for the offspring that similar letters are written next to each other to help sort the results.


6. To calculate results, each possible offspring combination should be examined to determine both genotype and phenotype. A typical monohybrid heterozygous cross

always yields a phenotype with a 3:1 ratio, whereas a typical dihybrid heterozygous cross always yields a phenotype with a 9:3:3:1 ratio.

What is meant by the terms sex determination, linkage, crossover, and mutation?

There are several factors that are important for livestock producers to understand.

Some of these are: A. Sex determination–Determination of

the sex of the zygote depends on the sex chromosomes. The process differs slightly among species.


1. Mammals–Male sex chromosomes are either X or Y. A zygote that receives a Y chromosome from

sperm will be male. A zygote that receives an X chromosome from

sperm will be female. The male determines the sex, as all eggs from the

female receive an X chromosome. Therefore, a female zygote will have two X

chromosomes (XX), whereas a male zygote will have one X and one Y chromosome (XY).


2. Poultry–The female determines the sex of the offspring. The male carries two sex chromosomes

(ZZ). The female carries only one sex

chromosome (ZW). After meiosis, all the sperm cells carry a Z

chromosome. Only half of the egg cells carry a Z

chromosome, the other half carries a W chromosome.


B. Linkage–The tendency for certain traits to appear in groups in the offspring is called linkage. Early studies in genetics were based on the

idea that all genes are redistributed in each mating.

It was found, however, that some groups of traits seemed to stay together in the offspring.


C. Crossover–The formation of new chromosomes resulting from the splitting and rejoining of the original chromosome is called crossover. This explains why the predicted results of a mating

do not always happen. During one stage of meiosis, the chromosomes line

up together. They are very close to each other. Sometimes the chromosomes cross over one

another and split. This forms new chromosomes with different

combinations of genes.


D. Mutation—The appearance of a new trait in the offspring that did not exist in the genetic makeup of the parents is called a mutation. Not all mutations are bad. In fact, some of the best discoveries in

genetics were unexpected mutations.

Review



animal, plant & soil science lesson b2-4 heritability of traits

Documents

heritability of certain

dominant traits

recessive traits

particular traits

heritability estimate

high heritability

recessive genes

pairs of genes