Genetics Terms
• Gene = a section of DNA (on a chromosome) that codes
for a specific trait
• Alleles = alternate forms of a gene
• Where are the alleles of a gene located?
• How many alleles can a person inherit for any one trait?
• How many alleles are there in a population for a particular
trait such as hair color?
Genetic terms, continued…
• Dominant allele =
• Homozygous dominant =
• Heterozygous =
• Homozygous recessive = aa
– Recessive trait is expressed
• Genotype = particular alleles a person carries: AaBBccDd
• Phenotype = an individual’s observable traits (hair color,
eye color, height,….
Fig 9.7
Gregor Mendel, the
father of genetics
•Each offspring inherits
two units (alleles) of
information, one from
each parent
Fig 9.6 a
Truebreeding
plants
(homozygous)
Fig 9.6b
All plants had purple flowers, all heterozygous
Monohybrid cross:
Pp x Pp
Punnett square
predicts a
phenotypic
ratio of 3:1 for
the F2
Generation
Mendel’s Principle of Segregation
Formation of gametes from a pre-gamete cell
Genotype of Pre-gamete cell: Aa
meiosis
Genotype of Gametes: A
a
The Chromosomal Basis
of Mendel’s Principle of
Segregation:
Pea shape gene
Round Wrinkled
Seed color gene
allele (R) allele (r)
Yellow
Green
allele (Y)
allele (y)
Nucleus
Mendel’s Second question: how do 2 pairs of genes (on
different chromosomes) assort into gametes?
–
Mendel’s 2nd
experiment:
Fig 9.08
Fig 9.08
9
3
3
1
Mendel’s principle of independent
assortment
• =
(figure not in book)
R
r
R
r
Y
y
y
Y
R
r
R
r
Y
y
y
Y
RY
ry
Ry
rY
Laws of Probability—application to inheritance
1. The results of one trial of a chance event do not affect
the results of later trials of that same chance event
–
E.g. Tossing of a coin, gender of children, etc.
Laws of Probability—application to inheritance
2. The Multiplication Rule:
a. What are the chances of a couple having 4 girls?
b. E.g. What are the chances of a couple having a boy
with the following characteristics:
• Brown hair (3/4), Non-tongue roller (1/4), Blue eyes
(1/4), Attached earlobes (1/4)
How to Solve Genetics Problems
Sample Problem: Mom and dad are heterozygous
for tongue rolling where tongue rolling is
dominant to non-rolling. What is the chance that
the couple will produce a child that is a nonroller?
Use the following steps as a general guide to solve this
and other problems:
1. Select a letter to represent the gene involved:
2. Write the genotypes of the parents.
3. Determine all possible gametes for each parent.
4. Make a Punnett square to represent all possible gamete
combinations between the two parents and determine
the genotypes of the offspring.
5. Use the genotypes found in the Punnett Square to
determine the possible phenotypes of the offspring to
answer the question.
Types of genetics problems: Monohybrid Cross
Sample problem: A true breeding black mouse was
crossed with a true breeding brown mouse to produce
the F1 generation, below. The F1 generation was then
inbred to produce an F2 generation.
a.) Which allele is dominant? How do you know?
b.) Determine the genotypes and phenotypes for all
3 generations
c.) Predict the genotypic and phenotypic ratios for
the F2
Monohybrid Cross Sample Problem #2
A mouse with black fur was crossed with a mouse with
brown fur to produce the F1 generation, below. The F1
generation was then inbred to produce the F2 generation.
Dominance is the same as in sample problem #2.
a.) Determine the genotypes and phenotypes for all 3
generations
b.) Predict the genotypic and phenotypic ratios for the
F2.
Monohybrid Cross Sample Problem #3
A couple, Jack and Jill, is concerned about having a child
with cystic fibrosis. Although both of Jack’s and both of
Jill’s parents are healthy and show no signs of cystic
fibrosis, both Jack and Jill each had a sister die of the
disease. The couple went to a clinic to be genetically tested
for cystic fibrosis and were each found to be heterozygous
for cystic fibrosis. What are the chances of Jack and Jill
having a….
a.) phenotypically healthy child?
b.) child that is homozygous dominant? Heterozygous?
Homozygous recessive?
c.) girl with cystic fibrosis? Boy with cystic fibrosis?
Types of Genetics Problems: Dihybrid Cross
Dihybrid Cross Sample Problem #1: True breeding
parental pea plants were crossed to produce the F1
generation, below. The F1 generation was inbred to
produce an F2 generation.
a.) Which alleles are dominant? How do you know?
b.) Determine the genotypes and phenotypes for all
3 generations
P:
Long & purple flowered pea plant x Short &
white flowered pea plant
F1: All Long & Purple Flowered pea plants
F2: 9 Long & Purple : 3 Long & White : 3 Short &
Purple : 1 Short & White
Genotype ratio for F2:
1/16 = YYRR
2/16 = YYRr
2/16 = YyRR
4/16 = YyRr
1/16 = YYrr
2/16 = Yyrr
1/16 = yyRR
2/16 = yyRr
1/16 = yyrr
Example
•
From the crosses below, what are the chances of
producing an organism with all
–
–
–
1.
2.
3.
dominant phenotypes?
recessive phenotypes?
homozygous dominant genotypes?
AaBb x AaBb
AaBbCc x AaBbCc
AaBBCc x aabbcc
How to use the probability method
1. Treat the problem as if it consisted of several
monohybrid crosses
2. Determine the gametes for each of these monohybrid
crosses
3. Make a Punnett square for each of the monohybrid
crosses
4. Use the information from each Punnett square and the
“multiplication rule” to solve the problem
Non-Mendelian Inheritance
• Sometimes there are more than 2 alleles in a
population
1. Codominance
• Both alleles expressed together as
heterozygotes
Codominance: Blood Types
Blood
Type
(Phenotype)
Surface
Molecule
on R.B.C.
Possible
Genotypes
A
IAIA or IAi
B
IBIB or IBi
AB
IAIB
O
ii
• Alleles
• IA = Allele for Type A
• IB = Allele for Type B
• i = Allele for Type O
• What do these alleles
code for? Different
carbohydrate “markers”
on the RBCs
Blood Types: Sample Problem #1
A couple has the type A and Type B, respectively.
Is it possible for them to have a child with the
following blood types?
a. Type O
b. Type A
c. Type B
d. Type AB
Blood Types: Sample Problem #2
A couple has the type A and Type AB,
respectively. Is it possible for them to have a child
with the following blood types? If so, what is the
genotype of each parent?
a. Type O
b. Type A
c. Type B
d. Type AB
Rhesus Factor—a RBC surface molecule
• Rh factor is inherited
independently from the
ABO system
• Rh positive people:
• Rh Negative people:
• Alleles
– R = Rh factor is present
– r = no Rh factor present
Phenotype
Rh +
(Rh positive)
Rh(Rh negative)
Possible
Genotypes
Blood Types: Sample Problem #3
A couple has the type A+ (heterozygous) and Type AB+,
respectively. What are the chances of the couple having a
child with the following phenotypes? Both parents are
heterozygous for the rhesus factor.
a. Type O+
b. Type Oc. Type A+
d. Type Ae. Type B+
f. Type Bg. Type AB+
h. Type AB-
2. Incomplete dominance
• One allele isn’t fully dominant over the
other
Fig 9.16
3. Continuous variation in a population
•
•
•
•
•
Individuals in a population show a range of
small differences in a certain trait
Ex.
Causes:
1.
2. Multiple environmental factors affect gene
expression.
Fig 9.21
1. Polygenic
inheritance:
aabbcc
AABBCC
2. Environmental influence on gene expression:
Ex.
Human Genetics
• Three types of disorders:
1. Autosomal recessive –
2. Autosomal dominant :
• AA – embryo dies
• Aa – is afflicted
• aa – normal individual
• Ex. Dwarfism
• Vary rare
3. Sex-linked recessive inheritance
• Recessive on X-chromosome
• e.g.
• Males usually get the disorder more than females –
why?
• Son cannot inherit a recessive allele from his father
but a daughter can. Why?
Human Sex Chromosomes
• Sex chromosomes in humans
– Female Genotype =
– Male Genotype =
• Sex-linked Alleles are carried on the X-chromosome
• Y-chromosome
– SRY gene on Y chromosome stimulates gonads to
differentiate into male sex organs.
• SRY = Sex-determining Region, Y-chromosome
Gender
Determination
in Humans
Female
Parents
Male
Chromosomes
segregate in
meiosis
Sperm
Offspring
Eggs
Two
daughters
Two
sons
Sample Problem
Mary’s paternal and maternal grandfathers are both
colorblind. There is no evidence of colorblindness
in either grandmother’s family histories, and
Mary’s father is not colorblind
a. What is Mary’s genotype? Phenotype?
b. What are the chances that Mary’s brother is
colorblind?
Other genetic diseases can arise through
changes in chromosome structure