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Transcript
Mendelian
Genetics
Genetics Lecture III
Biology Standards Covered



2c ~ students know how random chromosome
segregation explains the probability that a particular
allele will be in a gamete
2e ~ students know why approximately half of an
individual’s DNA sequence comes from each
parent
2g ~ students know how to predict possible
combinations of alleles in a zygote from the genetic
makeup of the parents
Biology Standards Covered


3a ~ Students know how to predict the probable
outcome of phenotypes in a genetic cross from the
genotypes of the parents and mode of inheritance
(autosomal or x-linked, dominant or recessive)
3b ~ Students know the genetic basis for Mendel’s
laws of segregation and independent assortment
Gregor Mendel

Thought of as the “founder” of modern
genetic research

He worked on an Austrian monastery in
the mid 1800’s

His work with the “garden peas”
explained many unanswered questions
about genetics
Important Vocabulary

Heredity – the passing on of
characteristics from parents to
offspring

Traits – characteristics that are
inherited (for example your eye color)

Genes – chemical factors that
determine traits
Important Vocabulary

Gametes – individual male and female
sex cells

Pollination – the transfer of pollen
(male gametes) to the pistil (female
part of a flower)

Fertilization – the uniting of male and
female gametes
Chromosome Number

Each organism on
this planet has a
specific number of
chromosomes

Humans (for example)
have 23 pairs of
chromosomes with a
total of 46

Apes have 24 pairs for
a total of 48

We share 99% of our
genome with chimps
and bonobos
Garden Pea plants

The garden pea plants were “self
pollinating” when Mendel first
observed them

This means that the offspring of each
pea plant was an exact copy of the
parent plant
Garden Pea plants

Mendel wanted to change the “exact
copy” offspring

He did so by “cross – pollinating”

He basically pollinated one pea plant
with a completely separate one
Genes and Dominance

Mendel found that when you make
certain “crosses” some of the traits
were hidden or masked

When he made further crosses some of
these traits came back
Genes and Dominance

This led Mendel to believe that there
were Dominant traits and less
dominant or recessive traits
The dominant trait is expressed by a
capital letter (for example D)
 The recessive trait is expressed by a
lower case letter (for example d)

Mendel’s Experiment

The original parent pea plants were called
the P generation
P Generation
Tall
Short
Mendel’s Experiment

The first set of offspring in the experiment is
called the F1 – Generation
P Generation
Tall
Short
F1 Generation
Tall
Tall
Mendel’s Experiment

The second set of offspring from the F1 –
Generation is called the F2 - Generation
P Generation
Tall
Short
F2 Generation
F1 Generation
Tall
Tall
Tall
Tall
Tall
Short
Alleles

Alleles are different forms of a gene
 By
“form” we mean the amount or variety of traits
 If
there are three different possible colors for flowers
on a pea plant, then there are three possible Alleles

For example:
a
TALL allele may be written as
a
short allele may be written as
T
t
Genotype vs. Phenotype

The Genotype is the “genetic makeup”
 Homozygous TALL plants would be: T T
 Heterozygous TALL plants:
Tt
 Homozygous small:
tt
* The recessive trait is “short” and in order for
the plant to be short both alleles must be
lowercase (recessive)
Genotype vs. Phenotype

The Phenotype is the “physical expression” of
the gene

Dominance Rule:

If a dominant allele is present in the Genotype, the
Phenotype will show the dominant trait
T is a dominant allele for TALL
 T T will yield a TALL plant
 T t will also yield a TALL plant
* The ONLY way a short plant will occur
is if BOTH alleles are recessive!! (tt)

F1 Generation Cross
Tt x Tt Cross




This diagram shows a
cross between the F1 –
generation
Both plants being
crossed are
Heterozygous
That is; they have both
alleles in their genotype
What would be the
outcome?
Probability in Genotypes

In a Heterozygous cross



1 out of 4 will be
homozygous dominant
(25%)
2 out of 4 (1/2) will be
heterozygous dominant
(50%)
1 out of 4 will be
homozygous recessive
(25%)
Mendel’s Principles

Biological characteristics are inherited as genes
from parents to offspring

Some forms (alleles) of genes are dominant and
others are recessive

In most sexually reproducing organisms, each adult
has two copies of a gene – one from each parent

The alleles for different genes usually segregate
independently from each other
Exceptions to Mendel’s Principles



Some alleles are
neither dominant nor
recessive
Many traits are
controlled by multiple
alleles or genes
Incomplete Dominance –
when there isn’t a
dominant allele at all

This appears as a third
allele (red, white, & pink)
Exceptions to Mendel’s Principles

Codominance – a case in which both alleles
contribute to the phenotype of the organism

If a red and white flower were crossed, codominance
would result in a red flower with white stripes or white
with red stripes


Both alleles are expressed at the same time in the offspring
Multiple Alleles – if a gene has more than two
alleles

This means that more than two possible alleles exist in
a population
Multiple Alleles
Exceptions to Mendel’s Principles

Polygenic traits – when a trait is controlled by two
or more genes that can be found on similar or
completely different chromosomes
Human skin color is controlled by more than four individual
genes
 Human eye color is also considered polygenic
