Title: Theoretical Genetics
1Theoretical Genetics
Gregor Mendel
2Objectives
- 4.3.1 Define dominant allele, recessive allele,
codominant alleles, locus, homozygous,
heterozygous, genotype, phenotype, carrier,
and test cross. - 4.3.2 Determine the genotypes and phenotypes of
the offspring of a monohybrid cross using a
Punnett grid.
3Review of DNA Structure
- Eukaryotic DNA is divided among
several chromosomes (humans
have 23 pairs of chromosomes
one set from each parent). - Chromosomes are divided into units called
genes. - Genes code for the bodys proteins eye pigment.
- Genes come in versions
called alleles - Eye color gene either brown or blue
allele. - Result of mutation.
4Definitions
- Locus
- The particular position of a gene on homologous
chromosomes.
Homologous codes for the same things.
5Definitions
- Homozygous vs. heterozygous alleles
- An organism with two identical alleles for a
character is homozygous for that character. - Organisms with 2 different alleles for
a character are heterozygous.
Bb BB bb
6Definitions
- Dominant recessive allele (complete dominance)
- A dominant allele is strong and is expressed (or
seen) in the phenotype. (Abbreviated with
capitals) - A recessive allele (if present in a
heterozygous individual) is
not expressed it is hidden because
it is weak. Reces- sive alleles only
show up if the in- dividual is
homo- zygous. (Lowercase
letters)
Bb BB bb
7Definitions
- Incompletely dominant alleles
- Sometimes, neither allele (when heterozygous)
can overpower the other. They produce a blend.
allele for white flower color (Incomplete
dominance) allele for red flower color
(Incomplete dominance)
Pink Red
White flowers flowers flowers
8Definitions
- Codominant alleles
- Pairs of alleles that both affect the
phenotype when present in a
heterozygote. - Ex Blood groups
- A, B, O are alleles that code
for a sig- nal
protein on the
membrane surface. O is
recessive, but A B
codominant.
Punnet square
9Definitions
- Genotype vs. phenotype
- All the many alleles of an organism (for eye
color, hair color, seed appearance, etc.) make
up its genotype genetic make-up think type of
genes. - An organisms physical characteristics make a
phenotype.
10The science of genetics
- Genetics - the study of heredity.
- Heredity a characteristic of life the passing
of traits from parents to offspring. - Traits - characteristics such as eye color, body
size, sickle cell anemia, etc. - The science of genetics be-
gan with an Austrian monk named
Gregor Mendel in the 1860s,
working with pea plants
(Pisum sativum).
11The science of genetics
- Mendel studied pea flower, seed pod color, and
seed shape. - Did careful pollination work counted offspring.
- Ex he mated plants with white flowers to plants
with purple flowers, or plants with wrinkled
seeds to plants with smooth seeds (the parental
generation, or P).
12The science of genetics
- Mendels work with peas
- Mendel found that plants of the first
generation (F1) were often identical. - Ex all had purple flowers or
all had yellow seeds. ?? - But mating the first generation (F1) plants
gave a 31 ratio in the 2nd generation (F2). - F stands
- for filial (son)
- F1 generation
F1 generation
13Mendels Law of Segregation
- A pair of allelic genes for a particular
character, like eye color, separate (segregate)
in equal ratios during gamete formation. - During meiosis, alleles are separated on a 11
ratio into sperm and eggs - Ex if the genotype Gg, half the sperm must
contain G and the other half must contain g.
14Punnet grids
- To make a Punnet grid
- Determine the geno- type of the
parents. - Make every possible
combination of gamete. - Combine the sperm
and the egg. - Determine the phenotypes from
the genotypes of
the offspring.
A monohybrid cross one locus is considered
Male parent Female parent
Sperm
Eggs
P represents the gene that makes the purple
flower pigment. The allele p is mutated, and the
protein is defective. All F1 offspring are Pp and
purple.
15Punnet grids
F1 generation
- To make a Punnet grid
- Determine the geno- type of the
parents. - Make every possible
combination of gamete. - Combine the sperm
and the egg. - Determine the phenotypes from
the genotypes of
the offspring.
F2 generation
16Punnet grids
- Incomplete dominance
- Ex red and white snapdragon flowers
- Blending of color
- A 121 ratio of
offspring
17Test cross
- To test an unknown individual
- Testing a suspected heterozygote by
crossing it with a known homozygous
recessive individual. - 1 of 2 possible outcomes
shows the genotype of
the unknown
parent.
18Objectives
- 4.3.3 State that some genes have more than two
alleles (multiple alleles). - 4.3.4 Describe ABO blood groups as an example
of codominance and multiple alleles.
19Mendels Law of Independent Assortment
- Alleles of genes on nonhomologous chromosomes
assort independently during meiosis. - All blonds do not have blue eyes.
- Hair color eye color are on different
chromosomes.
Chromosomes are shuffled.
20Punnet grids
- To make a dihybrid cross
- (crossing 2 pairs of genes)
- From parents genotypes,
determine gametes.
every possible combination
Make every possible combination of
gametes. (independent assortment)
21Punnet grids
- To make a dihybrid cross
- From parents genotypes, determine gametes.
- Combine the sperm and eggs, and determine
phenotypes.
22Multiple alleles
- Codominant alleles
- Pairs of alleles that both affect the
phenotype when present in a
heterozygote. - Ex Blood groups
- A, B, O are alleles that code
for a sig- nal
protein on the
membrane surface. O is
recessive, but A B
codominant.
Punnet square
23Multiple alleles
- Some genes have more than two alleles.
- ABO blood group shows codominance of multiple
alleles (IA and IB are codominant i is
recessive).
O (ii) is the universal donor AB accepts any
blood.
24Punnet square
- Punnet square for blood groups
- I is dominant, but there are 2 types IA IB
- i is recessive.
Here, a mother with blood group O mates with a
father whos AB.
25Punnet square
- Punnet square for sex-linked traits
- For genes found on a sex chromosome, X or Y.
- Remember, boys have only one X chromosome, so
they are more likely to get these diseases, like
color-blindness or hemophilia.
Notice, 50 of babies are boys, and 50 are
girls. Girls get 2 Xs, so they are likely to
have a good back-up copy and dont get affected.
26Multiple alleles
- Polygenic inheritance
- Two alleles on each of three genes have
an additive effect
on skin
color.
27Genetic diseases
- Sickle-cell anemia
- The gene for hemoglobin is mutated.
- An individual with two copies of the recessive
allele cant move oxygen around the body well.
28Genetic diseases
- Sickle-cell anemia
- A carrier is an individual that has one copy of a
recessive allele that causes a genetic disease in
individuals that are homozygous for this allele. - Carrier genotype Hhs - resistant to malaria
The gene for hemo- globin (H) is mutated.
Both Hhs
HH
hshs
Hhs
Homozygous recessive individuals often die young.
29Sex-linkage and Pedigrees
30Diseases linked to sex chromosomes
- Sex-linked traits genes are on the sex
chromosomes - XX individuals are female
- XY individuals are male.
Theoretically 5050 malefemale. Actually 5149
malefemale at birth then 5050 by age 3 (boys
weaker).
31Diseases linked to sex chromosomes
- Color-blindness is caused by a defective gene for
a pig- ment receptor in the eye. Gene is on X
chromosome. - Boys only get 1 X, so they are more likely than
girls to get this problem.
32Diseases linked to sex chromosomes
- Color-blindness is caused by a defective gene for
a pig- ment receptor in the eye. Gene is on X
chromosome. - Boys only get 1 X, so they are more likely than
girls to get this problem.
Retina of the eye
33Diseases linked to sex chromosomes
- Hemophilia a genetic disease in which the body
does not produce sufficient amounts of a clotting
factor. As a result, fibrin (necessary to
maintain the blood clot) does not form, so the
individual is more likely
to bleed long- er from a
wound, and to bleed
internally. - The gene for this clotting factor is on the
X chromo- some.
Boys have no back-up copy of the X.
34Genetic diseases - sex linkage
- Hemophilia genotypic phenotypic ratios
The female carrier is heterozygous
Note the daughters can be either
heterozygous for sex-linked diseases or
homozygous if a carrier mother marries a diseased
man. 1 in 10,000 males is affected with
hemophilia, and 1 in 100,000,000
females.
35Genetic diseases
- Hemophilia A case study
- Alexei, tsarevich of Russia inherited the
disease from his mother.
The original mutation in this lineage
occurred in Queen Victoria.
36Pedigrees
- X-linked dominant trait passed from the father
- Only his daughters get the disease.
37Pedigrees
- Y-linked trait (carried on the Y chromosome).
- Only boys get the disease.
38Karyotype
- A karyotype is a picture of the bodys
chromosomes - It shows abnormalities, also the individuals sex.
39Karyotype
- A karyotype is a picture of the bodys
chromosomes - It shows abnormalities, also the individuals
sex. - Down Syndrome three copies of chromosome 21.