Genetics
Ch. 11 pgs. 263-274
What is Genetics?
• The study of heredity
• Passing of traits from one generation to
the next.
Gregor Mendel (1822)
• Monk who worked in garden of monastery
in Czech Republic.
• Used pea plants to study
heredity
• Known as the “father
of modern genetics”
Why Pea Plants?
•
•
•
•
•
Many contrasting traits
Reproduce sexually
Crosses can be controlled
Large # of offspring
Short lifecycles
A little more about pea plants…
• Normally self pollinating
• Pollen (sperm) fertilizes the ovule (egg) to
produce offspring.
– Produce genetically identical offspring.
• Mendel used cross-pollination in his
experiments.
• Used pollen from one plant to fertilize another.
Genes and Dominance
• Mendel studied 7 traits of pea plants
– Traits are specific characteristics
– For each trait there are 2 contrasting
characteristics.
Generations
• P- parental generation; original pair
• F1- first filial generation; offspring from P
generation
• F2- second filial generation; offspring from
F1
Mendel’s Findings
• When he crossed P generation, F1
showed only one trait from the 2 parents.
– Ex. Crossed Tall x Short short or tall
Mendel’s Conclusions
• Biological inheritance is determined by
factors passed from one generation to the
next.
– Factors are called genes.
– 2 contrasting forms of the same trait are
called alleles.
• Ex. Trait is height, alleles are tall and short
Mendel’s Conclusions
• Principal of dominance- Some alleles are
dominant and some are recessive.
– Dominant- when allele is present organism
will always exhibit the dominant trait.
– Recessive- only exhibit recessive trait if
dominant allele is not present.
Alleles
• Symbols for alleles:
– A letter symbol is used for a specific trait.
• Ex- height, Tt
• Dominant- symbol is uppercase letter, T
• Recessive- symbol is lowercase letter, t
Segregation
• When Mendel crossed plants from the F1
generation traits from the P generation
reappeared.
P Tall x Short all tall plants
F1 Tall x Tall some tall some short
Law of Segregation
• The 2 alleles for each trait must separate
when gametes form.
– Parent only passes one
allele for each trait to its
offspring.
Probability and Punnett Squares
• The principles of probability can be used
to predict outcomes of genetic crosses.
• Punnett square- diagram
which shows possible
Results of genetic crosses
Heterozygous vs. Homozgous
• Hetero different
• Homo same
• Homozygous dominant- 2 identical
dominant alleles (upper case). Expresses
the dominant trait.
– Ex- TT, tall
Heterozygous vs. Homozygous
• Homozygous recessive- 2 identical
recessive alleles (lower case). Expresses
the recessive trait.
– Ex- tt, short
• Heterozygous- Have 2 different alleles for
the same trait. ( upper and lower case)
Express the dominant trait.
– Ex. Tt, tall
Genotype vs Phenotype
• Genotype- genetic make-up, letters
– Tt vs tt
• Phenotype- physical
expression of the trait
– Tall vs short
Sample Problem
• In summer squash, white fruit color (W) is
dominant over yellow fruit color (w). If a
squash plant homozygous for white is
crossed with a plant homozygous for
yellow what will the phenotypic and
genotypic ratios be?
Sample Problem
• If 2 heterozygous white plants are crossed
what will be the phenotypic and genotypic
ratios?
Types of Genetic Crosses
• Monohybrid one trait
– Ex. Tt x tt
• Dihybrid 2 traits
– Ex- Hair and height
– B= black, b=blonde, H=tall, h=short
– What would be the phenotypes of a cross
between the following parents?
HhBb x HHbb
Types of genetic crosses cont.
• Ex 2- Shape and color of pea plants
R= round, r=wrinkled, Y= yellow, y=white
What would be the phenotypes of a
cross between a plant that is RRYY and a
plant that is rrYy?
Genetics Review
• If normal vision (N) is dominant to
colorblindness (n) what are the
chances that a heterozygous normal
man and a colorblind woman will
have a child with colorblindness?
Genetics Review
• In some flowers red flowers (R) are
dominant to white flowers (r) and tall
stems (T) are dominant to short
stems (t). What is the genotypic ratio
for a cross between the following
plants:
RrTT x rrTT
DD:DR:RD:RR
Types of genetic crosses cont.
• Incomplete dominance
– One allele is not completely dominant over
the other
– Heterozyous phenotype is somewhere
between the 2 homozygous phenotypes.
• 3 phenotypes instead of 2
– Use all capitol letters
• Ex; RR= red, WW= white, RW= pink
Types of genetic crosses cont.
• Inc. Dom. Example:
In snapdragons the combination of red and
white flowered plants can produce a pink
flowered plant.
RR= red
WW= white RW= pink
What would be the phenotypes of the offspring if
a red flower were crossed with a pink flower?
Types of genetic crosses cont.
• Co-dominance
– Both alleles contribute to the phenotype
– Ex- BB= black feather, WW= white feather,
BW= black and white spotted feathers.
– Same rules as inc. dom. but both phenotypes
appear separately
Types of genetic crosses cont.
• Co-dominance sample problem
We have 2 fuzzy bunnies in class. One has
black and white fur, the other is pure white. What
are the genotypes of both rabbits?
Black and White=
White=
What would be the phenotypes of the offspring if
these rabbits mated?
Blood Type
• Human blood type is determined by the
type of protein found on the red blood cells
(A or B).
• Antigen- protein located
on blood cell
• Antibody- protein found in
plasma, prevent foreign
particles
Blood type
Blood
type
Possible
genotype
Antigen on RBC Antibodies in
Blood
Type that can
be received
A
A
B
A, O
B
A
B, O
AB
iAiA
iAi
iBiB
iBi
iAiB
A&B
None
O
ii
None
A&B
AB, A, B,
O
O
B
Blood Type Example
• What are the chances of a mother with A
type blood and a father with O type blood
having a child with A type blood?
Polygenic Inheritance
• Some traits are polygenic- have more than
one gene coding for the trait.
– Ex- skin color, eye color, height
Polygenic Inheritance cont
• 1. The weight of a fruit in a certain variety
of squash is determined by two pairs of
genes: AABB produces fruits weighing 4
lbs each and aabb produces fruits
weighing 2 lbs each.
A.
How many pounds does each
dominant allele add to the total weight of an
individual squash?
Human Genetics
• Chromosomes and Sex Determination
– Humans have 23 pairs of chromosomes in
autosomal cells. (diploid)
• In sex cells there are only 23. (haploid)
• Sex chromsomomes are the only pair that
are non-homologous.
– XX female
– XY Male
Human Genetics
• Ex- Cross between
male and female for
sex chromosomes:
– Chance of boy:
– Chance of girl:
Sex-Linked Inheritance
• Some traits are linked to the X
chromosome.
• Most genetic disorders are X-linked.
• Most X-linked disorders are expressed in
males.
WHY???
Sex- Linked Inheritance
• Females usually act as carriers, they carry
the recessive trait on one of their X
chromosomes but the trait is not
expressed.
Sex-Linked Inheritance
• Ex- Normal colored vision (N) is dominant
to colorblindness (n). Colorblindness is an
X-linked trait.
– Males are colorblind more then women and
the gene is only found on the X chromosome.
What is the chance that a man with normal
vision and a woman who is a carrier will have
a child that is colorblind?
Autosomal Traits
• Genetic traits that are carried on the
autosomal chromosomes.
Autosomal Recessive
• Only expressed with homozygous
recessive genotype.
• Heterozygous genotype
is carrier.
• If trait present usually
lethal disorder.
– Ex- cystic fibrosis
Autosomal Recessive
• ExC= no cystic fibrosis
c= cystic fibrosis
A female homozygous dominant for cystic
fibrosis marries a man heterozygous for
cystic fibrosis. What is the probability the
child will have cystic fibrosis?
Autosomal Dominant
• Expressed in homozygous dominant and
heterozygous genotype.
• No carrier (Either you have it or you don’t)
• If trait is present usually
lethal.
– Ex- Huntington’s Disease
Autosomal Dominant
• Ex
H= Huntington’s disease h= normal
A female who has Huntington’s disease and
is heterozygous for the disorder marries a
man who does not have the disorder.
What is the probability that their child will
have the disorder?
Pedigrees
• Chart which shows the relationships within
a family.
– Shows the expression of genetic traits.
• Symbols used:
– Circle=female
– Square=male
– Half-shaded=carrier
– Completely shaded=expresses trait
Pedigree
Pedigree
• Used to determine if disorder is autosomal
dominant, autosomal recessive or Xlinked.
– If only expressed in males and carried in
female X-linked
– If there are many carriers of both sexes
autosomal recessive.
– If no carriers and expressed in both sexes
autosomal dominant
Pedigree examples
Pedigree Examples
Pedigree Examples