Gregor Mendel studied traits in pea plants such as color and height. He found that traits are controlled by alleles - dominant alleles will be expressed over recessive alleles in the first generation. Through subsequent generations and self-pollination, he discovered that recessive traits can be expressed according to his laws of inheritance and principles of dominance and segregation. Mendel's experiments laid the foundations for modern genetics.

1. Mendel
• Gregor Mendel – born 1822 in Austria
• Monk in a monastery who tended the
garden
• Studied traits in pea plants (color and
height)

2. Pea Plant Experiment
• Pea plants self-pollinate – has both male
and female reproductive structures –
anther (male) and stigma (female)
• Self-pollinating plants are true breeding –
all offspring have the same traits as the
parent
• Cross-pollination occurs when one plant
fertilizes another plant (insects)

3. Characteristics
• Characteristics – a feature that has different
forms in a population
• Trait – different forms of characteristic
• Dominant trait – trait that appears
• Recessive trait – trait that recedes
• Fertilization – during sexual reproduction, egg
and sperm join
• First generation – first cross, dominant trait is
apparent
• Second generation – second cross, recessive
trait appears

4. Cont.
• Principles of dominance – some alleles
are dominant and some are recessive
• Hybrid – offspring of crosses between
parents with different traits

5. Traits
• Pea plants, like most organisms, carry two set of
instructions for each characteristic - dominant
and recessive traits
• These instructions are called genes
• Genes are part of DNA, each parent contributes
a set of these instructions to the offspring
• Offspring carries the two forms of the gene for
every characteristic
• Different forms of genes are called alleles

6. Expressing Alleles
• Genotype – combination of alleles an
organisms inherits from its parent
• Dominant alleles are always written in
capital letters (PP) (Pp) and are always
first in the series
• Recessive alleles are written in lowercase
letters (pp) and are second in the series

7. Expressing Alleles cont.
• Phenotype – an organisms appearance
• Expressed by the trait that shows up,
either dominant or recessive
• Heterozygous– have two different alleles
for same trait
• Homozygous – two identical alleles for
trait

8. Probability
• Probability – the mathematical chance that
something will happen
• Written as a fraction, ratio or percentage
• Punnett Square – tool used to calculate
the outcome of alleles

9. • Principle of independent assortment – genes
for different traits can segregate independently
during the formation of gametes, helps account
for the many genetic variations observed in
organisms
• Chromosomes sort independently not individual
genes
• Incomplete dominance – one allele is not
completely dominant
• Codominance – both alleles contribute to the
phenotype

10. • Multiple alleles – more than two alleles
(more than two possible outcomes)
• polygenic trait – trait controlled by two or
more genes
• Human body cells have 46 chromosomes
or 23 pair of homologous chromosomes –
a complete set
• Human sex cells have 23 chromosomes
(half the number of all other cells 46)

11. • 2 of the 46 are called sex chromosomes
• Other 44 are called autosomes
• Female – XX
• Male – XY
• All human egg cells carry a single X
chromosome (23,X), half of all sperm cells carry
an X chromosome (23,X) and half carry a Y
chromosome (23,Y) to ensure that just about
half of the zygotes will be 46,XX and half will be
46,XY

12. • The extra female X is randomly “switched
off” and resides in the Barr body
• Males have one X chromosome and all X-
linked alleles are expressed in males even
if it is recessive