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Section 7
Genetic Inheritance
Genes

Genes are parts of chromosomes that
carry information

Most organisms get half their genetic
information from one parent and half
from the other so different forms of
the same gene are called alleles
Genotype and Phenotype

Genotype describes the alleles present
for a characteristic

Genotypes are always described using
letters e.g. Bb, bb, BB

Phenotypes describe the outward
physical appearance of a genotype and are
always described using words e.g. Brown
or blue eyes
Alleles

Alleles can be dominant, recessive or codominant

Dominant alleles show up in the physical
appearance of an organism

Recessive alleles do not show up in the physical
appearance of an organism unless there are two
of them present

Co-dominant alleles both show up in the
physical appearance
Alleles
Two alleles that are the same is known as a
homozygous genotype

Further description can be added to see if
it dominant or recessive

AA would be homozygous dominant

aa would be homozygous recessive
Allele
Two alleles that are different are known as
a heterozygous genotype

No further description is needed here
because in a heterozygous genotype there
must be two forms of the same gene e.g.
Aa
True Breeding
Organisms which pass on characteristics in
a predictable way are said to be true
breeding

True breeding organisms are
homozygous
Word Test
1.
Different forms of the same gene.
2.
Describes the alleles present for a characteristic.
3.
Describe the outward physical appearance of a genotype and are always described
using words e.g. Brown or blue eyes.
4.
Alleles show up in the physical appearance of an organism.
5.
Alleles do not show up in the physical appearance of an organism unless there are two
of them present.
6.
Alleles both show up in the physical appearance.
7.
Two alleles that are the same.
8.
Two alleles that are different.
9.
Organisms which pass on characteristics in a predictable way.
True Breeding Monohybrid
Cross
True Breeding Monohybrid
Cross
Predictions Vs Reality

With true breeding parents, the expected
ratio in the F2 generation is normally 3:1

In reality, the expected numbers differ
from the actual numbers

This is because fertilisation is a
random process
Co-dominance
Sure,
why not
Red Cow
RR
White Cow
WW
Roan Cow
RW
Co-dominance

Two alleles of a gene can be co-dominant when neither is dominant
nor recessive.

Both alleles can be expressed

E.g. red cows crossed with white will generate roan cows. Roan
refers to cows that have red coats with white blotches.

When the F1 roan are self crossed, the F2 have a phenotypic
ratio of :
1 red:2 roan:1 white.

This type of inheritance is called incomplete dominance.
Make a baby!

The way people look is all down to random chance.

Having dominant, recessive or co-dominant characteristics is totally
dependant on which egg and sperm meet at fertilisation and which
genes these gametes were allocated during meiosis.

Today you are going to pair up with someone and “make a baby” by
carrying out the following instructions:
◦ Traits will be written on the board – the dominant, recessive or codominant version will be described.
◦ You cannot choose which trait your baby has, this will be determined
by rolling the dice. If you have an even number, it is dominant, odd
numbers are recessive.
◦ Work through all the characteristics and draw your baby. Have a look
at other couples’ babies. Do they look similar or different?!
Family Trees
P
F1
F2
Polygenic Inheritance
Poly = many
 Genic = genes
 “many gene” inheritance


Two or more genes control a characteristic

This means there are lots of different phenotypes

Examples include: hand span, height, weight, foot
length, leaf length
Continuous Variation

Continuous variation is when there are no
clear cut distinctions between groups

E.g. Weight: some people are 8st 1lb, some are
9st 12lb

There are NO distinct groups e.g. 8st or 9st
etc
CONTINUOUS VARATION = POLYGENIC INHERITANCE
Normal distribution graphs

Continuous variation can be measured
and plotted on a line graph or histogram:
Examples of continuous variation include:
 Height
 Weight
 Hand span
 Length of feet
 Milk yield in cows
Single Gene Inheritance

Characteristics are controlled by just one
gene

There are therefore only one of two
phenotypes

E.g. Tongue rolling or non-tongue rolling,
green or yellow seeds
Discontinuous Variation

There are clear-cut differences between
groups

E.g. Being able to roll your tongue or not;
you can either do it or you cannot
SINGLE GENE INHERITANCE = DISCONTINUOUS VARIATION
Bar graphs
Discontinuous
variations are
shown using bar
graphs
Number of plants
number of plants
9
8
7
6
5
4
3
2
1
0
white
pink
red
Flower colour
purple
Blood type