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Genetics
Chapter 12
12-1: Origins of Hereditary Science
Mendel’s Experiments


Genetics – The study of how characteristics
are transmitted from parents to offspring
Heredity – The passing on of genetic
characteristics from one generation to
another
Mendel’s Experiments

Gregor Mendel (an Austrian monk) used
cross-pollination techniques in which pollen is
transferred between flowers of two different
plants.
Mendel’s Experiments

Reasons Mendel used pea plants



Require little care
Produce many offspring
Mature quickly (8-10 weeks)
Mendel’s Experiments




P (parental) generation – Organisms that are
true-bred for specific traits
F1 generation – Offspring of the P generation
F2 generation – Offspring of the F1 generation
F = Filial (pertainting to a son or daughter)
Mendel’s Experiments


Self-pollination – A plant with both male and
female reproductive organs that fertilizes
itself
Cross-pollination – Pollen from one plant is
carried to the flower of another plant
Mendel’s Experiments

Three Steps of Mendel’s Experiments
Mendel’s Experiments

Gene – A section of
DNA that carries the
information on how to
make one protein
Mendel’s Experiments


Allele – An alternative
form of a gene
One allele for each trait
is passed from each
parent to the offspring.
Mendel’s Experiments

Dominant trait – An allele
that is fully expressed
whenever the allele is
present in an individual


The trait that masks the
other
Recessive trait – An allele
that is expressed only when
there is no dominant allele
present in an individual

The trait that is masked
12-2: Genetic Crosses
Monohybrid Cross


Monohybrid cross – A
cross in which one
characteristic is tracked
Punnett square – used
to predict the outcome
of genetic crosses
Monohybrid Cross


Genotype – the genetic makeup of an
organism (use letters)
Phenotype – the appearance of an organism
(physical appearance)
Monohybrid Cross


Homozygous – having identical alleles
Heterozygous – having different alleles
Monohybrid Cross

Trait – Plant height

Phenotype



T – tall
t – short
Genotype



TT – homozygous
dominant (pure bred)
Tt – heterozygous
(hybrid)
tt – homozygous
recessive (pure bred)
Monohybrid Cross

Probability – the likelihood that a specific
event will occur

Probability = number of one kind of possible outcome
total number of all possible outcomes

A probability may be expressed as a decimal, a
percentage, or a fraction
Mendel’s Characteristics
Monohybrid Cross

Ratios and percentages




TT x tt = 100% heterozygous
TT x TT = 100% homozygous dominant
tt x tt = 100% homozygous recessive
Tt x tt


50% heterozygous
50% homozygous recessive
Monohybrid Cross

Ratios and percentages

Tt x Tt = Genotypic ratio – 1:2:1 ratio

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
25% homozygous dominant
50% heterozygous
25% homozygous recessive
Tt x Tt = Phenotypic ratio – 3:1

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75% tall
25% short
Dihybrid Cross

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Dihybrid cross – A breeding experiment that
tracks the inheritance of two traits
Example


Traits: Pea shape & Pea color
Alleles: R – round
r – wrinkled
Y – yellow
y – green
Dihybrid Cross

Use the FOIL method for each parent to setup the Punnett square
RrYy x RrYy
RY Ry rY ry
RY Ry rY ry
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry RRYy
rY RrYY
ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
24
Why eye color changes in people
is unknown
The trait for eye color
actually has three
alleles:
AaBbCc
This chart is
used to
illustrate the
variety of eye
colors
Mendel’s Laws

The law of segregation – The two members
of a gene pair (alleles) segregate (separate)
from each other in the formation of gametes.

Half the gametes carry one allele, and the other
half carry the other allele.
Mendel’s Laws

The law of
independent
assortment – Genes
for different traits
assort independently
of one another in the
formation of gametes
Other Monohybrid Crosses

Incomplete dominance
– a form of intermediate
inheritance in which
one allele for a specific
trait is not completely
dominant over the other
allele

This results in a
combined phenotype.
Other Monohybrid Crosses

Codominance – Two alleles are expressed
(multiple alleles) in heterozygous individuals


Two alleles that are both dominant
Example: blood types

Key
A = IA
B = IB
O=i

A and B are dominant over O


Other Monohybrid Crosses
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Genotypes
Type A = IAIA or IAi
Type B = IBIB or IBi
Type AB = IAIB
Type O = ii
Phenotypes
AA or AO
BB or BO
AB
OO
Other Monohybrid Crosses

Sex-Linked traits – Traits (genes) located on
sex chromosomes



X – Female sex chromosome
Y – Male sex chromosome
Many sex-linked traits carried on X
chromosomes
Pedigree

Pedigree – A diagram of family relationships
that uses symbols to represent people and
lines to represent genetic relationships