Download Animal, Plant & Soil Science

Animal, Plant & Soil Science
Lesson B2-4
Heritability of Traits
Interest Approach
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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
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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
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adenine
alleles
chromosome
cytosine
DNA
dominance
double helix
eukaryotic
gametes
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gene expression
genes
genetic code
genetics
genome
guanine
heredity
incomplete
dominance
Terms
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alleles
codominance
crossover
dihybrid cross
dominant
genotype
heritability
heritability
estimate
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heterozygous
homozygous
incomplete
dominance
linkage
monohybrid
cross
mutation
Terms
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phenotype
probability
Punnett square
qualitative traits
quantitative traits
recessive
sex
chromosomes
How do we estimate the
heritability of certain traits?
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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.
How do we estimate the
heritability of certain traits?
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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.
How do we estimate the
heritability of certain traits?
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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.
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Their phenotype is either one thing or the other.
These traits most easily show how genes are
inherited.
An example is coat color.
How do we estimate the
heritability of certain traits?
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2. Quantitative traits are traits
controlled by several pairs of genes.
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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.
How do we estimate the
heritability of certain traits?
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3. Not all traits contained within an
organism are expressed.
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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.
How do we estimate the
heritability of certain traits?
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4. The Punnett square is a technique for
predicting genotype.
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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?
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Resulting offspring of reproduction
have both genotype and
phenotype heredity.
A. Genotype is the actual genetic
code.
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It controls physical and performance
traits.
The genotype of an organism cannot
be changed by environmental
factors.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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B. Phenotype is the organism’s
physical or outward appearance.
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This is the part of the
genotype the organism
expresses or shows.
In some instances,
phenotype may be
altered by the
organism’s environment.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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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.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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1. A monohybrid cross takes into account a
single trait and is represented by a single set of
letters.
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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.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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2. A dihybrid cross takes into account
two traits and would be represented by
four letters instead of two.
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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.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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3. A Punnett square is created by drawing a
four-square (two by two) or a 16-square (four
by four) box.
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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.
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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.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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5. To determine the genotype
and phenotype of the
offspring, the parent’s
offerings are combined and
written in the corresponding
inner squares.
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It is important when writing the
letters for the offspring that
similar letters are written next
to each other to help sort the
results.
How do we predict the genotypes and
phenotypes from monohybrid and dihybrid
crosses by using the Punnett square?
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6. To calculate results, each possible
offspring combination should be
examined to determine both genotype
and phenotype.
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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?
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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.
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The process differs slightly among species.
What is meant by the terms sex
determination, linkage, crossover, and
mutation?
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1. Mammals–Male sex chromosomes are
either X or Y.
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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).
What is meant by the terms sex
determination, linkage, crossover, and
mutation?
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2. Poultry–The female determines the
sex of the offspring.
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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.
What is meant by the terms sex
determination, linkage, crossover, and
mutation?
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B. Linkage–The tendency for certain
traits to appear in groups in the offspring
is called linkage.
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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.
What is meant by the terms sex
determination, linkage, crossover, and
mutation?
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C. Crossover–The formation of new
chromosomes resulting from the splitting and
rejoining of the original chromosome is called
crossover.
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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.
What is meant by the terms sex
determination, linkage, crossover, and
mutation?
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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.
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Not all mutations are bad.
In fact, some of the best discoveries in
genetics were unexpected mutations.
Review
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How do we estimate the heritability of
certain traits?
How do we predict the genotypes and
phenotypes from monohybrid and
dihybrid crosses by using the Punnett
square?
Review

What is meant by the terms sex
determination, linkage, crossover, and
mutation?