Genetics and
Heredity
(Mendelian)
History
• Genetics is the study of genes.
• Inheritance is how traits, or characteristics,
are passed on from generation to
generation.
• Chromosomes are made up of genes, which
are made up of DNA.
• Genetic material (genes,chromosomes,
DNA) is found inside the nucleus of a cell.
• Gregor Mendel is considered “The Father of
Genetics"
Genetics Notes
Who is Gregor Mendel? “Father of Genetics”
Principle of Independent Assortment – Inheritance of one
trait has no effect on the inheritance of another trait
Law of Segregation
• During the formation of gametes (eggs or
sperm), the two alleles responsible for a
trait separate from each other.
• Alleles for a trait are then "recombined"
at fertilization, producing the genotype for
the traits of the offspring.
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Applying the Law of Segregation
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Gregor Mendel
• Austrian Monk.
• Experimented with “pea plants”.
• Used pea plants because:
– They were available
– They reproduced quickly
– They showed obvious differences in the traits
Understood that there was something that
carried traits from one generation to the
next- “FACTOR”.
Mendel's Plant Breeding Experiments
Gregor Mendel was one of the first to apply
an experimental approach to the question
of inheritance.
For seven years, Mendel bred pea plants
and recorded inheritance patterns in the
offspring.
Particulate Hypothesis of Inheritance
Parents pass on to their offspring separate
and distinct factors (today called genes)
that are responsible for inherited traits.
What genetic principles account for the transmission of such traits from
parents to offspring?
The Blending Hypothesis of Inheritance
In the early 1800’s the blending hypothesis was proposed. Genetic material
contributed by the two parents mixes in a manner analogous to the way blue
and yellow paints blend to make green.
What would happen if this was the case?
Mendelian Genetics
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Dominant traits- traits that are expressed.
Recessive traits- traits that are covered up.
Alleles- the different forms of a characteristic.
Punnett Squares- show how crosses are made.
Probability- the chances/ percentages that
something will occur.
Genotype- the types of genes (Alleles) present.
Phenotype- what it looks like.
Homozygous- two of the same alleles.
Heterozygous- two different alleles.
Traits
• Genetics – study of how traits are passed from parent
to offspring
• Traits are determined by the genes on the
chromosomes. A gene is a segment of DNA that
determines a trait.
• Chromosomes come in homologous pairs, thus genes
come in pairs.
Homologous pairs – matching genes – one from female
parent and one from male parent
• Example: Humans have 46 chromosomes or 23 pairs.
One set from dad – 23 in sperm
One set from mom – 23 in egg
Alleles: alternative versions of a gene.
The gene for a particular inherited character resides at a specific locus
(position) on homologous chromosome.
For each character, an organism
inherits two alleles, one from each
parent
How do alleles differ?
Dominant allele
Recessive
allele
Recessive allele
Recessive allele
Dominant - a term applied to the trait (allele) that is expressed regardless of
the second allele.
Recessive - a term applied to a trait that is only expressed when the second
allele is the same (e.g. short plants are homozygous for the recessive allele).
Dominant and Recessive Genes
• Gene that prevents the other gene from “showing” –
dominant
• Gene that may NOT “show” even though it is present –
recessive
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
Dominant
color
Recessive
color
Example: Straight thumb is dominant to hitchhiker thumb
T = straight thumb t = hitchhikers thumb
(Always use the same letter for the same alleles—
No S = straight, h = hitchhiker’s)
Straight thumb = TT
Straight thumb = Tt
Hitchhikers thumb = tt
* Must have 2 recessive alleles
for a recessive trait to “show”
• Both genes of a pair are the same –
homozygous or purebred
TT – homozygous dominant
tt – homozygous recessive
• One dominant and one recessive gene –
heterozygous or hybrid (carrier)
Tt – heterozygous
BB – Black
Bb – Black w/
white gene
bb – White
Genotype and Phenotype
• Combination of genes an organism has (actual gene
makeup) – genotype
Ex: TT, Tt, tt
• Physical appearance resulting from gene make-up –
phenotype
Ex: hitchhiker’s thumb or straight thumb
Punnett Square and Probability
• Used to predict the possible gene makeup of offspring –
Punnett Square
• Example: Black fur (B) is dominant to white fur (b) in mice
1. Cross a heterozygous male with a homozygous recessive female.
Black fur (B)
Heterozygous
male
White fur (b)
White fur (b)
Homozygous
recessive female
White fur (b)
To test the particulate hypothesis, Mendel crossed truebreeding plants that had two distinct and contrasting traits—for
example, purple or white flowers.
What is meant by “true breeding?”
Mendel cross-fertilized his plants by hand. Why is it important to control
which plants would serve as the parents?
For each monohybrid cross, Mendel cross-fertilized true-breeding plants that
were different in just one character—in this case, flower color. He then allowed
the hybrids (the F1 generation) to self-fertilize.
Typical breeding experiment
P generation (parental
generation)
F1 generation (first filial
generation, the word filial
from the Latin word for
"son") are the hybrid
offspring.
Allowing these F1
hybrids to self-pollinate
produces:
F2 generation (second
filial generation).
It is the analysis of this
that lead to an
understanding of genetic
crosses.
Mendel studies seven characteristics in the garden pea
Law of Dominance
In the monohybrid cross (mating of two organisms that differ in only one
character), one version disappeared.
What happens when the F1’s are crossed?
Probability and Punnett Squares
Punnett square: diagram showing the probabilities of the
possible outcomes of a genetic cross
Genotype versus phenotype.
How does a
genotype ratio differ
from the phenotype
ratio?
Punnett squares - probability diagram illustrating the possible
offspring of a mating.
Ss X Ss
gametes
Testcross
A testcross is designed to reveal whether an organism that displays the
dominant phenotype is homozygous or heterozygous.
Sex Determination
• Humans – 46 chromosomes or 23 pairs
• 22 pairs are homologous (look alike) – called autosomes – determine body traits
1 pair is the sex chromosomes – determines sex (male or female)
• Females – sex chromosomes are homologous (look alike) – label XX
Males – sex chromosomes are different – label XY
• What is the probability of a couple having a boy? Or a girl?
Chance of having female baby? 50%
male baby? 50%
X
X
X
XX
XX
Y
XY
XY
Who determines the sex of the child? father
Dihybrid Cross
• A breeding experiment that tracks the
inheritance of two traits.
• Mendel’s “Law of Independent
Assortment”
• a. Each pair of alleles segregates independently
during gamete formation
• b. Formula: 2n (n = # of heterozygotes)
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Summary of Mendel’s laws
LAW
DOMINANCE
SEGREGATION
INDEPENDENT
ASSORTMENT
PARENT
CROSS
OFFSPRING
TT x tt
tall x short
100% Tt
tall
Tt x Tt
tall x tall
75% tall
25% short
RrGg x RrGg
round & green
x
round & green
9/16
pods
3/16
pods
3/16
pods
1/16
pods
round seeds & green
round seeds & yellow
wrinkled seeds & green
wrinkled seeds & yellow
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