This document provides an overview of genetics concepts including: - Chromosomes contain DNA made of genes that code for proteins and control traits. - Alleles are variants of genes that can be dominant or recessive and determine traits. - Mendel's experiments with pea plants showed dominant and recessive inheritance through monohybrid and dihybrid crosses modeled with Punnett squares. - Independent assortment of chromosomes and random fertilization create genetic variation.

1. +
Genetics Recap

2. +
Objectives
To be able to:
 Explain how characteristics are inherited from parents and how
genetic variations occur.
 Demonstrate the characteristics that will be inherited by
offspring using punnett squares.
 Differentiate between a monohybrid cross & a dihybrid cross.

3. +
The cell’s nucleus contains chromosomes.
These are long threads of DNA, each
made up of many genes
A gene is a section of DNA
that codes for a specific
protein.

4. +
Some characteristics, such as eye colour and
the shape of the earlobe, are controlled by a
single gene. These genes may have different
forms.
Different forms of the same gene are called
alleles. The gene for eye colour has an allele
for blue eye colour and an allele for brown eye
colour.

5. +
Alleles
Alleles are dominant or recessive:
• the characteristic controlled by a dominant allele
develops if the allele is present on one or both
chromosomes in a pair
• the characteristic controlled by a recessive allele
develops only if the allele is present on both
chromosomes in a pair

6. +
A monohybrid cross shows the inheritance of one
characteristic. In the genetic diagrams for these crosses:
• the recessive allele is represented by a lower case
letter
• the dominant allele is represented by an upper case
letter
• someone with two identical copies of an allele is
homozygous for that particular gene
• someone with two different alleles for a particular gene
is heterozygous for that gene

7. +

8. +
Mendel’s experiment
Gregor Mendel (1822-1884) studied the inheritance of
different characteristics in pea plants. He found that
when he bred purple-flowered plants with white-flowered
plants, all the offspring produced purple flowers.
If he bred these plants with each other, most of the
offspring had purple flowers, but some had white. This
was because the allele for purple flowers is dominant,
and the allele for white flowers is recessive.

9. +
Punnett Squares
A punnett square shows how chromosomes or
alleles may combine in zygotes.

10. + Mendel’s first cross

11. +
homozygous dominant
X
homozygous recessive
=
all heterozygous, all dominant
(100% heterozygous dominant)
Mendel’s first cross

12. + Mendel’s second cross

13. + Mendel’s second cross
heterozygous X heterozygous
= 1:2:1
homozygous dominant:
heterozygous: homozygous
recessive
(3:1 = dominant : recessive)

14. +
A dihybrid cross shows the inheritance of two
characteristics.
In a dihybrid cross the genes that determine the two
characteristics are located on different pairs of homologous
chromosomes. Each of these genes can have two or more
alleles.

15. +
YR Yr yR yr
YR
Yr
yR
yr
Y = yellow
y = green
R = round
r = wrinkled

16. +

17. +
A dihybrid cross involves two traits.
 Mendel’s dihybrid crosses with heterozygous plants
yielded a 9:3:3:1 phenotypic ratio.
• Mendel’s dihybrid crosses
led to his second law,
the law of independent
assortment.
• The law of independent
assortment states that
allele pairs separate
independently of each
other during meiosis.

18. +Heredity patterns can be calculated
with probability.
 Probability is the likelihood that something will happen.
 Probability predicts an average number of occurrences, not an exact number of
occurrences.
• Probability =
number of ways a specific event can occur
number of total possible outcomes
• Probability applies to random
events such as meiosis and
fertilization.

19. + Sexual reproduction creates
unique combinations of genes.
 Sexual reproduction creates unique combination of genes.
 independent assortment of chromosomes in meiosis
 random fertilization of gametes
 Unique phenotypes may give a reproductive advantage to some
organisms.

20. + Crossing over during meiosis
increases genetic diversity.
 Crossing over is the exchange of chromosome segments between
homologous chromosomes.
 occurs during prophase I of meiosis I
 results in new combinations of genes

21. • Chromosomes contain many genes.
– The farther apart two genes are located on a chromosome, the more likely
they are to be separated by crossing over.
– Genes located close together on a chromosome tend to be inherited
together, which is called genetic linkage.
• Genetic linkage allows the distance between two genes to be calculated.