KEY
Study Guide
Topic # 3033
Genotype and Phenotype
Anna Blight
Define genotype.
Genotype is the genetic constitution, or makeup, of an individual.
What is the difference between genes and alleles?
A gene is a small segment of chromosome (DNA) that is a genetic code for a
trait and determines how the trait will develop.
An allele is a gene that occupies corresponding loci on each chromosome pair
and affects the same trait for that gene. Each loci may carry a different allele
and determines dominant or recessive traits.
For any pair of alleles, how many genotypes are possible?
Three, AA, Aa, and aa.
Define homozygous and heterozygous and give an example of each.
Homozygous is the term for an individual whose alleles of the genes for a
particular trait are the same, for example, AA and aa.
A heterozygous individual has two different alleles for a trait, for example,
Aa.
What do the terms dominant and recessive genes mean?
A dominant gene overpowers and prevents expression of its recessive allele
when the two alleles are present in a heterozygous individual. A recessive
trait will only be expressed when both alleles are recessive, also called
homozygous recessive.
Define phenotype and how it relates to genotype.
Phenotype is the physical characteristics of an animal that can be seen and/or
measured (e.g., the presence or absence of horns, the color, or the weight of
an animal). A dominant genotype (AA or Aa) or homozygous recessive
genotype (aa) is expressed phenotypically.
Explain genotypic ratios and compare to phenotypic ratios.
Genotypic ratios compare the number of homozygous dominant,
heterozygous, and homozygous recessive individuals. For example, if two
heterozygous animals are crossed (Aa x Aa) the result will be a 1 : 2: 1 ratio.
There will be 1 homozygous dominant, 2 heterozygous, and 1 homozygous
recessive. For this cross, the phenotypic ratio is 3 : 1; 3 dominant expressions
to 1 recessive expression.
How is the Punnett square used?
The Punnett square is used to graph the two alleles for a gene of one animal
on the top line of the square, and the two alleles of a gene for the other parent
along the side of the square. When the animals are crossed, the genotypes of
the offspring are shown in the center of the square.
Explain how the genetic code is passed on.
The genetic code is passed from the DNA to the RNA. The RNA then
transcribes the code into proteins made up of amino acids. During formation
of egg and sperm (gametogenesis), only one allele of each gene is contained in
each gamete. When reproduction takes place, each gamete contributes one
allele to the zygote, which will have the combined genetic code of both
parents.