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Genetics (10.2, 10.3, Ch.11)
SB2. Students will analyze how biological traits are passed on to successive generations.
(c) Using Mendel’s laws, explain the role of meiosis in reproductive variability.
(e) Compare the advantages of sexual reproduction and asexual reproduction in different
situations.
Some Needed Vocabulary
Heredity
Passing on of characteristics from parent to
offspring
Traits
Characteristics that are inherited
Genetics
The study of Heredity
Gametes
Sex cells
Sperm = male gamete
Egg = female gamete
Gregor Mendel
A monk who studied heredity
using garden pea plants
Why garden peas?
They reproduce sexually
They have both male & female gametes on the
same plant
What did he do?
He transferred pollen from one plant to another
with different traits
This is called “making a cross”
Mendel first “crossed” tall plants with short plants
Mendel’s Laws
Law of segregation
The two alleles for each trait must
separate when gametes are formed
A parent will pass down AT RANDOM,
only one allele for each trait
Law of independent assortment
Genes for different traits are inherited
independently of one another
Mendel’s Monohybrid crosses
Monohybrid cross
Focuses on one (mono) trait
1st generation
Crossed a tall pea plant with a short pea plant
All of the offspring were tall
Mendel’s Monohybrid crosses
2nd generation
Crossing the offspring of the 1st generation
¾ of the offspring were tall; ¼ of the offspring
were short
3:1 ratio (tall to short)
Mendel’s Findings
Mendel found that each
chromosome has two factors for
each traits… called alleles
Alleles = forms of genes
Types of alleles
Dominant
The allele that has the ability to mask the
other
Represented with a capital letter (A)
Recessive
The allele that will be masked by the
dominant allele… it will be there, but
will not be expressed
Represented with a lowercase letter (a)
Phenotype vs. Genotype
Phenotype
The way that an offspring looks;
how the trait appears
Tall, short
Genotype
The offspring’s genetic
combination
TT, Tt, tt
Homozygous vs. Heterozygous
Homozygous (“homo” means “same”)
Having two identical alleles for a trait
TT, tt
Heterozygous (“hetero” means “different”)
Having two different alleles for a trait
Tt
Punnett square
Short way to find the expected
proportions of possible genotypes in
the offspring of a testcross
Monohybrid cross vs. dihybrid cross
Monohybrid cross
Involves the cross of
ONE trait
Uses a 4 block
punnett square
Dihybrid cross
Involves the cross of
TWO traits
Uses a 16 block
punnett square
“F.O.I.L.” to produce
gametes
Complete Dominance
Inheritance characterized
by an allele that is fully
expressed in the
phenotype of a
heterozygote and that
masks the phenotypic
expression of the
recessive allele
Example:
In humans, dimples are dominant over
no dimples. What is the probability of
producing a dimpled child from two
heterozygous dimpled parents?
Exceptions to Mendel’s Law
Mendel’s traits showed two distinct forms
Most genes do not exhibit simple inheritance
Genotypic ratios persist but phenotypic ratios may vary
because of interactions between
Alleles
Other genes
Segregation of genes on the same chromosome
Environment
Lethal Alleles
Some allele combinations are lethal
Must be taken into consideration when completing
punnett squares
Incomplete Dominance
Pattern of inheritance in
which the dominant
phenotype is not fully
expressed in the
heterozygote, resulting in a
phenotype intermediate
between the homozygote
dominant and homozygous
recessive
Example:
In snapdragons (a type of plant), pink
and white petal colors show
incomplete dominance. What is the
likelihood of producing a red flower if
two pink flowers are crossed?
Codominance
Inheritance
characterized by full
expression of BOTH
alleles in the
heterozygote
Example:
In birds, specifically chickens, black
and white feather color is expressed in
a codominant fashion. What is the
likelihood of producing a black bird if
a white bird and black bird are
crossed?
Multiple Alleles
Some genes may have more than just two alternative forms of a
gene
The inheritance of the ABO blood groups is an example of a
locus with three alleles
Multiple Alleles
Example:
In humans, one of the ways blood types are determined is with a trait controlled
by multiple alleles. What is the likelihood of producing a child with type O blood
from two heterozygous parents one with type A blood and one with type B blood?
Epistasis
Interaction between two
non-allelic genes in which
one modifies the
phenotypic expression of
another
In horses, brown coat color
(B) is dominant over tan
(b).
Gene expression is
dependent on a second gene
that controls the deposition
of pigment in hair.
The dominant gene (C) codes
for the presence of pigment
in hair, whereas the recessive
gene (c) codes for the
absence of pigment.
Polygenic Inheritance
Mode of inheritance in which the additive effect of
two or more genes determines a single phenotypic
character
Sex-Linked Alleles
Controlled by genes located on
sex chromosomes (XX or XY)
Usually carried on X
chromosome
Since females are XX, they are
usually carriers of the trait
Since males are XY, they have a
higher tendency for
inheritance and EXPRESSION
of the trait
Example:
In humans, there is a genetic condition
known as Hemophilia that is
controlled by sex-linked inheritance.
What is the likelihood of having a
female carrier if a hemophiliac dad
and a normal mom are crossed?