Biol 3054
1.
Worksheet 6
Define the following terms.(do not use the word you are defining in the definition!!!)
Genotype – the sets of alleles present in the genome of an individual
Allele – a particular version of a DNA sequence present at a specific region of a chromosome (locus). When
considering genes, an allele is a version of gene which corresponds to a particular form of a trait
Phenotype – the outward, physical or biochemical manifestation of the genotype – the alleles of a gene which are
present in an organism
Dominant (do not use the word recessive in definition) – refers to a phenotypic condition which appears whether the
genotype is heterozygous or homozygous. An allele is dominant if it masks the second allele present in the
genome
Recessive (do not use the word dominant in definition) - refers to a phenotypic condition which appears only when
the genotype is homozygous for a given allele. An allele is recessive if the phenotype it causes is masked by
another allele present in the genome
Homozygous – the condition when the alleles at a locus on homologous chromosomes are identical
Heterozygous - the condition when the alleles at a locus on homologous chromosomes are different
Homologous chromosomes – chromosomes that have the same size, centromere position and nearly identical DNA
sequences
Meiotic recombination – The exchange of chromosome sequences between non-sister chromatids of homologous
chromosomes
Locus – position of a gene on an chromosome
Co-dominance – when two phenotypes are expressed simultaneously because equally, but distinct functional
enzymes are encoded by two alleles
Incomplete dominance – the condition resulting from a difference in dosage of proteins expressed. When a single
functional allele is insufficient to produce enough of a gene product to generate a phenotype intermediate to that
observed when two functional alleles are present.
Monohybrid cross – a mating between individuals that differ in phenotype of single variable trait for the purposes of
establishing dominance or recessiveness.
Dihybrid cross - a mating between individuals that simultaneously differ in phenotypes of two variable traits for the
purpose of establishing independent assortment or linkage
Biol 3054
2.
Worksheet 6
Two genetic laws are derived from the work of Gregor Mendel. These laws describe the way that genes are
assorted and arranged during gametogenesis. Using the two pairs of homologous chromosomes diagrammed
below, show how the alleles B, b, A, a could be distributed into the four gamete cells following meiosis. State
the two laws derived from Mendel’s work and how each law applies to the distribution of alleles in the diagram.
gametes
BA
Chromosome 1’
Ba
Chromosome 1
B
b
2 2’
A
1
bA
a
1’
ba
Chromosome 2
Chromosome 2’
1st Law – segregation of alleles – the two alleles governing a particular trait separate during gametogenesis so that
only one allele of each gene is present in a gamete
In the example above, the B and b alleles are always found individually in the gametes
2nd Law – independent assortment - either allele of one gene may be placed in combination with either allele of any
other gene
In the example above, the B allele can be in a gamete in combination with either the A or the a allele.
3.
Read the section on codominance on pg 199.
A. Bertha has type A blood and her husband Clyde, has type B blood. Draw a pedigree for Bertha & Clyde
using this information: Bertha’s father had blood type A and her mother had blood type O. Clyde’s father
had blood type O and his mother had blood type AB.
Bertha’s genotype is determined from the information given
about her parents. Her mother’s genotype must be ii, while
her father could be either IA iO or IA IA - it doesn’t matter.
What matters is that Bertha MUST have inherited an i allele
from her mother and the IA from her father. Clyde’s father
likewise passed on an i allele to him so Clyde must have the
IBi genotype.
type A
AO , AA
type O
OO
type AB
AB
type A
AO
B. Draw a Punnett square showing the mating between
Clyde & Bertha. From your Punnett square determine
what percentage of their children would be expected
to have type A, type B, type O or type AB blood.
type B
BO
IB
i
i
ii – type O
IB I – type B
IA
i IA – type A
IA IB – type
AB
25% type O, 25% type B, 25% type A, 25% type AB
type O
OO
Biol 3054
4.
5.
Worksheet 6
Pure-breeding, black mice with long tails are mated to pure-breeding, white mice with short tails. The resulting
F1 progeny are all black with short tails.
a. Which traits are dominant: _black coat_ & __short tails___
b.
Which traits are recessive: __white coat_ & __long tails___
c.
Assign designations for each allele for the two genes controlling the two traits observed in this cross.
black coat ____B____ ; white coat ____b_____; long tails ___s______; short tails ____S_____
d.
Male and female F1 siblings were mated. What proportion of their offspring would be predicted to have
each of the following phenotypes:
BbSs x BbSs
black, long tails _3/16 __
while, long tails __1/16__
black, short tails __9/16__
white, short tails __3/16__
Read the section on incomplete dominance on pg 198. A cross between rabbits with 20cm ears and rabbits with
10 cm ears resulted in F1 offspring that all had 15cm long ears. Predict all the phenotypes and proportion of
each that would result from a cross between a male and female F1 rabbit.
Parents: 20/20 x 10/10
F1: 20/10
F1 x F1 20/10 x 20/10
¼ with 20 cm ear, ½ with 15 cm ears, ¼ with 10cm ears