2, 3
321-324
How genes contribute towards the expression of traits and
characteristics
What kinds of offspring can there be?
Outcomes
Identify and explain:
 The possibility of genotypes of children from parents with specific alleles;
 How alleles for dominant or recessive phenotype are passed onto children from
parents;
 Ways in which scientists work out the proportion of children with certain possible
genotypes;
 What monohybrid is;
 What dihybrid is; and
 What a test cross is.
Students should able to:
 Use punnet squares to conclude the proportion of possible genotypes resulting from
the combining of parental alleles;
 Appreciate the importance of understanding the role that alleles play in passing on
genetic information and thus affecting the formation of genotypes in children.
Content
What kinds of children are possible?
 How can scientists estimate what characteristics and traits the offspring will have?
 The alleles that parents pass onto their children are determined by the chromosomal
sorting that occurs during gamete formation by meiosis. Non-homologous
chromosomes behave independent of each other during meiosis.


An example:
The TYR gene on the number-11 chromosome, controls pigment production.
During meiosis, the pair of number-11 chromosomes disjoin carrying the alleles to
different gametes. This separation of the alleles of one gene into different gametes
that occurs during meiosis is known as the segregation of alleles. The diagram below
shows the segregation of alleles. A represents dominant allele for pigmentation and
a represents recessive allele for non-pigmentation. In this diagram, Tracey is
heterozygous (Aa) with one dominant allele and one recessive allele; John is the
same.
The probabilities for children with certain types of characteristic and traits are
incorporated into a Punnett square:
The AA homozygote and Aa heterozygote will result in normal pigmentation,
whereas aa genotype will result in albinism. The fraction shows the chance in which
each genotype is likely to occur.
Combining probabilities: Two genes



The disjoining of one pair of chromosomes is independent of the disjoining of the
other pairs.
This independent disjoining ‘juggles’ the gene combinations in a process termed
gene assortment.
A dihybrid cross can be considered as a combination of two mono-hybrid cross (or
seen as a Punnet square).
For example: cross AaIi x Aali
Or in Punnet Square-form:
Test crosses
 A cross involving a single gene is a monohybrid cross (e.g. Aa x Aa)
 A cross involving two genes is a dihybrid cross (e.g. AaBb x AaBb)
 A cross that can either be monohybrid OR dihybrid is called a test cross.
 A test cross involves one parent that is homozygous recessive (e.g. aa or aabb) and is
used for the following purposes:
o To identify if an organism showing a dominant trait is homozygous or
heterozygous; and/ or
o To establish linkage relationships between genes.
 However, with the technology we have today, the above tests can be dome using
molecular techniques.
Activity
Application questions:
(1) The ABO gene on the number-9 chromosome controls the ABO blood types. Looking at
the diagram below, what possible genotypes can the children have? What are the
chances of each genotype occurring?
(2) CBD is a gene that controls colour vision (located on the X chromosome), in which an
allele for the dominant phenotype will result in production of green sensitive pigment
and an allele for the recessive phenotype will result in the lack of product of pigment
(colour vision defect). Amanda is heterozygous whereas Riley is homozygous recessive.
What possible genotypes can the children have? What are the chances of each
genotype occurring?
(3) RH is a gene that controls Rhesus blood type, in which an allele for the dominant
phenotype will result in a person being Rhesus positive. Page and Simon are both
heterozygous. What possible genotypes can the children have? What are the chances of
each genotype occurring?
(4) DMD is a gene that controls muscle protein and is located on the X chromosome. An
allele for the dominant phenotype will result in the production of normal muscle
protein. Sally and Harry are both homozygous dominant. What possible genotypes can
the children have? What are the chances of each genotype occurring?