5.2: Studying Genetic Crosses
pg. 208
Every cell carries two alleles for each trait. During Meiosis there
are two possible outcomes
Analyzing Genetic Crosses: Punnett Squares
Reginald Punnett (1875 – 1967) devised a visual technique to help
analyze the results of genetic crosses.
The punnett square use Mendel’s law of segregation to illustrate all
of the possible offspring that could be formed from the gametes of
the parents. From the punnett square, the probability of inheriting
genotype and phenotype percentage outcome can be determined.
There are two gametes produced by each parent, and each gamete
containing one possible allele. The probability of produce either
gamete is ½.
A Punnett Square Analysis of Mendel’s Experiments
A monohybrid cross is used to study only one trait at a time.
Using a punnett square, you can visually determine the possible
outcomes between a cross of two parents.
Figure 5.6: this Punnett square analysis provides the probability of each genotype and
phenotype in the F2 generation. The ratio of phenotypes in the F2 generation for this
example is 3:1. Therefore, it predicts that three purple flowered plants will form for
everyone white flowered plant.
Test Crosses: Determining the Genotype of a Parent That Has
a Dominant Phenotype.
To determine the genetic make up of an individual that expresses
dominant traits (phenotype), but the genotype is unknown, a test
cross is performed to determine the genotype.
The unknown genotype is crossed with a homozygous recessive
genotype. After analyzing the phenotypic ratio of the outcome
from the cross, the unknown genotype can be determined.
Test cross: is a cross between a parent of unknown genotype and a
homozygous recessive parent.
Figure 5.7: In a test cross, if any of the offspring show the recessive phenotype, the
unknown genotype of the parent must be heterozygous.
Working with Punnett Squares
Using punnett squares and tally charts will help determine the
probability of outcomes, and specific ratios for various crosses.
Practice Problems, questions 1 – 10, pg. 212
Learning Check, questions 7 – 12, pg. 212
The Inheritance of Two Traits: Dihybrid Crosses
Mendel designed a second set of experiments that involved
following two traits.
Dihybrid cross: is a cross of two individuals that differ in two traits
due to two different genes.
In one of Mendel’s experiments the P generation was true
breeding yellow, round with green, wrinkled. The F1 generation
expressed dominance for both traits; (yellow, round). When a F1
generation was allowed to interbreed or self pollinate, the F2
generation expressed the recessive forms.
Figure 5.8: One of Mendel’s dihybrid crosses involved true-breeding pea plants with
yellow, round seeds and true-breeding pea plants with green, wrinkled seeds. In the F2
generation, the ratio of plants with yellow, round seeds to plants with yellow, wrinkled
seeds, to plants with green, round seeds, and to green, wrinkled seeds, was 9:3:3:1.
Developing the Law of Independent Assortment
After performing many dihybrid crosses, when he crossed two
heterozygous parents for both traits, the outcome always produced
a 9:3:3:1 ratio.
A Punnett Square Can Model Mendel’s Results
Using the FOIL method to determine possible gametes for each
parent in F1 generation, there are four possible outcomes.
When a parent, with four gametes is crossed with a parent with
four gametes, the F2 generation, will have 16 outcomes and four
phenotypes and 9 genotypes.
Phenotypes:
9 displaying the dominant forms for both traits
3 displaying one dominant form and one recessive form for each trait
3 displaying the other dominant/recessive pair of traits
1 displaying both recessive forms of each trait
Figure 5.9: In the F2 generation, individuals in the largest group (9) have at least one
dominant allele for each gene (Y_R_). In the intermediate groups (3), the individuals
have at least one dominant allele for one gene, but 2 allele’s recessive alleles for the other
gene (Y_rr, yyR_). The smallest group is homozygous recessive for both genes (yyrr).
Learning Check, questions 1 – 16, pg. 215
Practice Problems, questions 11 – 20, pg. 216
The Chromosome Theory of Inheritance
Chromosome Theory of Inheritance: the traits determined by
genes are inherited through the movement of chromosomes during
meiosis.
Mendel studied pea plants and formulated his laws of inheritance,
with very little knowledge of how these traits are passed on. It was
to the 1900’s that meiosis was being determined and the
movement of chromosomes.
Walter Sutton (1877-1916) studied the process of segregation of
homologous chromosomes and their migration during meiosis (I
and II).
He determined that the movement of these chromosomes and the
formation of gametes followed the patterns determined by
Mendel’s law of segregation. The theory provided the basis for the
segregation and independent assortment.
Figure 5.10: Alleles (or Mendel’s factors) and chromosomes both segregate during
meiosis. During anaphase I, the homologous chromosomes segregate and move to
opposite ends of the cell. After telophase I, the homologous chromosomes are in separate
cells. The resulting gametes are equally likely to contain each possible combination of
alleles.
Review Questions, questions 1 – 12, pg. 218