Chapter 9 Observing Patterns in Inherited
Gregor Mendel wrote about “units of inheritance” we now know that
genes are the fundamental units of inheritance. It is essential for you
to make the connection between the movement of chromosomes in
meiosis and patterns of inheritance of genes carried on them.
Know the connection between Fig. 8.7 and 8.8. How does the
movement of chromosomes during the production of gametes in both
parents give rise to the given offspring?
***Do NOT study 9.5 and 9.8. NOT ON EXAM
Look at Genetics problems
You are required to be able to construct and understand the
resulting genotypes and phenotypes and their ratios for a complete
Punnet square for a Monohybrid and a Dihybrid cross.
EX: construct the Punnet square for the monohybrid cross
Aa x Aa
1 9.2 Tracking Traits pg. 151
Mendel started to study inheritance using the pea plant pg. 151
Mendel’s work tracking pea plant traits led him to conclude that
hereditary information passes from one generation to the next.
Look at Fig. 9.2 on page 151 purple flower peas crossed with white
flower peas will give you only purple flower peas
dominant – refers to an allele that masks the effect of a recessive allele
in heterozygous individuals
recessive – refers to an allele with an effect that is masked by a
dominant allele in heterozygous individuals
heterozygous – having 2 different alleles of a gene
homozygous – having identical alleles of a gene
genotype – the particular set of alleles that is carried by an individual’s
phenotype – An individual’s observable traits
monohybrid cross – cross between two individuals identically
heterozygous for one gene; Example Aa x Aa
Punnett square – Diagram used to predict the genetic and phenotypic
outcome of a cross
1. Individuals with identical alleles are homozygous for the allele
2. Heterozyous individuals have two non identical alleles
3. A dominant allele masks the effect of a recessive allele on the
4. Genotype (an individual’s particular set of alleles) gives rise to
phenotype (what the individual will look like
2 Take Home Message
How do alleles contribute to traits?
1. Genotype refers to the particular set of alleles that an individual
carries. Genotype is the basis of phenotype, which refers to the
individual’s observable traits.
2. A homozygous individual has two identical alleles of a gene. A
heterozygous individual has two non identical alleles of the
3. A dominant allele masks the effect of a recessive allele paired
with it in a heterozygous individual.
3 9.3 Mendelian Inheritence Patterns pg 152
Dihybrid cross – Cross between two individuals identically
heterozygous for two genes for example AaBb X AaBb
Monohybrid cross – cross between two individuals identically
heterozygous for one gene; for example Aa X Aa
1. Crossing individuals that breed true for two forms of a trait
yields identically heterozygous offspring. : A cross between such
offspring is a monohybrid cross.
2. The frequency at which the traits appear in the offspring of such
crosses can reveal dominance relationships among the alleles
associated with those traits.
3. Punnett squares are useful in determining the probability of the
genotype and phenotype of the offspring of crosses.
4. Diploid cells have pairs of genes on homologous chromosomes.
5. The two genes of a pair separate from each other during meiosis,
so they end up in different gametes.
6. Crossing individuals that breed true for two forms of two traits
yields offspring that are identically heterozygous for alleles
governing those traits : A cross between such offspring is a
7. Gene pairs on homologous chromosomes tend to sort into
gametes independently of other gene pairs during meiosis.
Take Home Message
How do alleles contribute to traits?
1. Diploid cells have pairs of genes, on pairs of homologous
chromosomes. The two genes of a pair (which may be identical or
not) are separated from each other during meiosis, so they end up
in different gametes
2. In most cases, the two genes of a pair are distributed into gametes
independently of other gene pairs during meiosis.
4 9.4 Behond simple dominance pg. 156
codominance – effect in which two alleles are both fully
expressed in heterozygous individuals and neither is dominant
over the other ***Example