Genetics

1
Genetics
2
What is Genetics?

• The branch of biology that seeks to explain
  biological variation
• Heredity Transmission of characteristics
  (traits) from parent to offspring

3
Vocab You Should Recall

• Diploid-
• a cell with 2 sets of homologous chromosomes
  (2n)
• Haploid-
• a cell with 1 set of chromosomes (n) without a
  homologous pair
• Gamete-
• a male or female sex cell (n)
• Zygote-
• a fertilized egg cell (2n)

4
More to Recall

• Chromosome
• a long piece of DNA containing many alleles
• Autosome
• All chromosomes not involved in determining sex
• Gene
• The length of DNA that codes for a trait.
• Genes come in pairs that separate in the
  formation of gametes.

5
New Vocab

• Genotype
• An organisms genetic make-up (allele combination)

• Phenotype
• - The physical appearance of a gene (visible
  trait)

6
What is the relationship between genes (genotype)
and observable characteristics (phenotype)?

• Phenotype Genotype Environment.

Genetically identical hydrangeas growing in soils
of different acidity (different environments).
The phenotype genotype environment principle
applies equally to human traits.
7
Different Genotypes Can Produce the Same Phenotype
8

• Allele
• Alternative versions of a gene (one from each
  parent 2 alleles one gene)
• Homozygous
• Having a pair of identical alleles for a
  characteristic (pure)
• Heterozygous
• Having 2 different alleles for a characteristic
  (hybrid)

• For example, the gene for seed shape in pea
  plants exists in two forms
• one form or allele for round seed shape (R)
• the other for wrinkled seed shape (r).

• A homozygous plant would contain the following
  alleles for seed shape
• RR or rr.
• A heterozygous plant would contain the alleles Rr

9
Classroom Genetics
10

• Dominant-
• In heterozygote, the allele that is expressed in
  phenotype
• Recessive
• In heterozygote, the allele that is completely
  masked in the phenotype

Earlobes Free ear lobes (dominant trait)
Attached ear lobes (recessive trait)
Dimples Dimples (dominant trait) No dimples
(recessive trait)
Tongue-Rolling Rolling up edges (dominant
trait) not rolling (recessive)
11

• Monohybrid Cross
• a genetic cross between individuals differing in
  one trait

• Dihybrid Cross
• - a cross between individuals differing in two
  traits

12

• F1 Generation
• The first generation of hybrid offspring in a
  genetic cross
• F2 Generation
• Offspring resulting from interbreeding of the
  hybrid F1 generation.

13

• Test Cross
• Breeding of recessive homozygote with dominant
  phenotype, but unknown genotype
• Punnett square-
• diagram used by biologists to predict the
  possible outcome of a genetic cross

14
Mendelian Genetics

• Gregor Mendel (1822- 1884)
• Known as the Father of Modern Genetics
• Austrian Monk (and HS science teacher) who
  wondered how plants obtain atypical
  characteristics
• Wrote Experiments with Hybrid Plants

15
Mendels Experiments..
16
Gregor Mendels Rules of Inheritance

• Rule of Dominance and Recessiveness
• The allele that expresses itself in the phenotype
  when a gene is heterozygous (hybrid) is the
  dominant allele. The allele that is masked is
  the recessive allele. Ex. Tongue rolling.
• Rule of Incomplete Dominance
• When a gene is heterozygous, incomplete dominance
  (or co-dominance) results when the phenotype is a
  mixture of the two genotypes. Ex. Red, pink and
  white snapdragons.
• Rule of Segregation
• During Meiosis, two alleles of a gene separate
  during the formation of gametes (egg and sperm).
• Rule of Independent Assortment
• Alleles of one gene separate independently of
  the alleles of any other gene. In other words,
  the way in which one pair of alleles segregates
  has nothing to do with the way any other pair of
  alleles segregate.

17
Nondisjunction

• Failure of homologous chromosomes to separate in
  meiosis

18

• Web Karyotyping activity

19
Punnett Square

• Predicts offspring genotype from parental gamates
  in a visual form

20
Tongue Rolling

• Dolly cannot roll her tongue, so we know she is
  homozygous recessive for that trait.
• Phenotype non roller
• Genotype tt
• Dollys dad cannot roll his tongue either
• Phenotype non roller
• Genotype tt
• Dollys mom can roll her tongue.
• Phenotype roller
• Genotype TT or Tt
• In order to have a daughter who cannot roll her
  tongue, what does Dollys mom genotype have to be?

In order for Dolly to be a non-tongue roller, her
mom has to be heterozygous for tongue rolling
Tt
Tt
-t
tt
-t
tt
t
21
Incomplete Dominance

• gene expression in which the phenotype of a
  heterozygous individual is intermediate between
  those of the parents.

22
Some general rules for genetics problems

• Two alleles are necessary for a trait
• The genes are symbolized by the first letter of
  the dominant gene.
• The letter for the dominant gene is always
  capitalized.
• The letter for the recessive trait is always
  lower case (make sure you can tell the difference
  between the two)
• Wild Type is the typical form of the organism,
  strain, or gene
• Pure traits are those with identical genes
  (homozygous).
• Hybrids have mixed genes for the same trait
  (heterozygous).
• Gametes only carry one allele for each trait
  (they are haploid)

23
Probability

• Mathematic tool used for predicting the
  likelihood of events
• Equal to the expected frequency of a particular
  event when an experiment is repeated an infinite
  number of times
• Probabilities in genetics are often predicted
  based on a hypothesis, which is tested with real
  data
• Mendel predicted outcomes of pea plant crosses
  and then tested them thousands of times over 8
  years!

24
Probability times event is expected to
happen opportunities (trials)

• usually expressed as a fraction (or ).
• Ex.
• The chance of a coin landing heads up is one out
  of two or ½ (50)
• The chance of drawing an ace out of a deck of
  cards is 4 out of 52 or 1/13 (7.7)

25

• Why can the principles of probability be used to
  predict the outcomes of genetic crosses?

Segregation of alleles in meiosis uniting of a
certain egg and sperm is random, like a coin
flip.
26

• Cystic fibrosis is an autosomal recessive
  disorder.
• To have the disease, an individual has to be
  homozygous recessive (ff)
• A man and woman are both carriers for cystic
  fibrosis (Ff). What are the chances of them
  having a child with the disorder?

FF
Ff
Ff
ff
27
First Law of Probability

• the results of one chance event have no effect on
  the results of subsequent chance events.
• ex. My cousin already has 3 boys. His wife is
  pregnant. What is the probability of his fourth
  child being a boy?
• ½

If my friend Victoria and her husband Peter are
carriers of cystic fibrosis and have already had
a son with the disorder, what are their chances
of having another child with the disorder? ¼ or
25
28
Two Other Rules of Probability youll need to
understand

• Rule of Multiplication-
• the probability of a compound event is equal to
  the PRODUCT of the separate probabilities of the
  independent single events
• prob(a and b) p(a)p(b)

Ex. The probability of flipping a coin and
getting 2 tails in a row
The probability of flipping tails on the 1st
penny ½
The probability of flipping tails on the 2nd
penny ½
The probability of getting tails on both pennies
¼

x
29
Rule of Addition-

• The probability of an event that can occur in two
  or more alternative ways is the SUM of the
  separate probabilities of the different ways.

Ex. The probability of flipping 1 head and 1
tail in a toss with 2 coins
The probability of flipping tails on the 1st
penny and heads on the 2nd penny (½)(½) ¼
The probability of flipping heads on the 1st
penny and tails on the 2nd penny (½)(½) ¼
The probability of getting one tail and one head
in a toss with 2 coins ½


30
Practice Problems

• What is the probability of any couple having 4
  boys in a row?
• ½ x ½ x ½ x ½ 1/16 or 6.25

What is the probability of my friend Victoria and
her husband having two kids with cystic
fibrosis? ¼ x ¼ 1/16
31
Is the following sentence true or false?
The past outcomes of coin flips greatly affect
the outcomes of future coin flips.
False
32

• How can you be sure of getting the expected 50
  50 ratio from flipping a coin?

You must flip the coin many times.
Therefore, the _____ the number of offspring from
a genetic cross, the closer the resulting numbers
will get to expected values.
GREATER
33
How to determine the expected outcome of a coin
toss

• a heads. On a coin, there is a ½ probability
  of tossing heads
• b tails. On a coin, there is a ½ probability of
  tossing tails
• When tossing coins simultaneously, use binomial
  expansion to determine all the possible
  outcomes/frequencies.

34
Chi Square (X2)

• How well do the observed results fit with the
  expected?
• X2 indicates the degree of deviation, tells you
  the of cases in which such a deviation might be
  expected by chance.
• The higher the X2 Probability (P value from
  table), the more likely that the results are
  true. The lower the X2 P, the more likely
  theres something askew (coin weighted
  differently on each side, sticky side, etc)

35
Degrees of Freedom

• of possibilities 1
• For a coin, you can toss heads or tails
• 2 possibilities
• Degrees of freedom would be 2-1 1
• For a die, the degrees of freedom would be
• 6 1 5

36
Dihybrid Crosses

• In this type of cross, each trait is considered
  separately (just as in a monohybrid cross)
• How many traits are involved in a dihybrid cross?
  
• 2
• How many pairs of genes are involved?
• 2
• How many total genes are involved?
• 4
• How many genes for each trait are found in
  gametes?
• 1
• How many total genes are involved in each gamete?
  
• 2

37
Practice 1

• A tall green plant is crossed with a short yellow
  plant. All of the offspring are tall and green.
• Genotypes of the parents
• Genotypes of the gametes

38
Practice 2

• A green wrinkled pea plant is crossed with a
  yellow round pea plant. All of the offspring
  have green round peas. Show how genes will
  arrange to do this
• Genotypes of the parents
• Genotypes of the gametes

• Genotype and phenotype of offspring
• genotype
• Phenotype

39
Practice 3

• If a plant that is hybrid for green and pure for
  wrinkled peas is crossed with a yellow wrinkled
  pea plant, predict the genotypes and phenotypes
  of the offspring.
• Genotypes of the parents
• Genotypes of the gametes

• Genotype and phenotype of offspring
• genotype
• Phenotype

40
Rule of Independent assortment

• The random selection of one trait will not
  determine the random selection of another
• In other words, the genes for your eyes are
  transmitted independently of the genes for your
  height

• See the dihybrid example to the right
• As you can see, there are 4 possible outcomes.
• One letter does not affect the selection of the
  other.

41
What Works for Peas Also Works for Humans

• Consider a cross between parents heterozygous for
  both deafness and albinism.

This is the same 9331 ratio seen for Mendels
cross involving pea color and shape.
42
Non-Mendelian Genetics

• i.e. exceptions to Mendels Rules
• Covers traits that have a range of phenotypes,
  not just 2 possibilities

43
Incomplete Dominance

• pattern of gene expression in which the phenotype
  of a heterozygous individual is intermediate
  between those of the parents.

44
INCOMPLETE DOMINANCE contd

• Yellow coat color in guinea pigs is produced by
  the homozygous genotype YY
• Cream color by the heterozygous genotype Yy
• White by the homozygous genotype yy.
• What genotypic and phenotypic ratios are matings
  between cream-colored individuals likely to
  produce?
• 121
• yellow cream white

45
Codominance

• the condition in which both alleles in a
  heterozygous organism are expressed.
• Ex. Roan horses or cows

46
CO-DOMINANCE contd

• In cattle, reddish coat color is not completely
  dominant to white coat color. Heterozygous
  individuals have coats that are roan colored (ie.
  reddish, but with spots of white hairs).
• What would be the results of the following
  crosses
• a. red x roan
• b. white x roan
• c. red x white
• d. roan x roan

Genotype phenotype 11 RRRr 11 Red Roan 11
Rrrr 11 RoanWhite All Rr all Roan 121
RRRrrr 121 RedRoanWhite
47
Multiple Alleles

• genes with three or more alleles
• Ex. Blood types- there is an A, B, and O allele
  (IA, IB, i)
• (More on this when we discuss blood type
  genetics)

48
Polygenic Trait-

• when several genes influence one trait.
• Ex. Eye color, height, hair, and skin color

49
Pleiotropy

• one gene that affects more than one seemingly
  unrelated phenotype

50
Epistasis

• one gene masks the expression of a different gene
  for a different trait
• Ex. The gene for albinism masks the effects of
  genes for skin, hair, and eye color.

51
Epistasis contd

• In Labs, black is dominant to chocolate (B or b).
  Yellow is recessive epistatic (when present, it
  blocks the expression of the black and chocolate
  alleles) E or e.
• Determine the number of chocolate labs produced
  from a black female and a yellow male (BbEe x
  bbee)

52
Influence of Environment

• phenotype depends on conditions in the
  environment.
• Ex. Arctic fox, Siamese cats, height, and skin
  color

Temperature sensitive The cold extremities
(ears, nose, tail, and feet) express pigmentation
while the warm body does not)
53
Height in humans is polygenic. Each upper case
letter adds 3. Males (aabbcc) are 5 and females
(aabbcc) are 4 7. How tall would a male with
the genotype AaBbCc be? 59 A female?
54 Probability that mating with this male
female result in a child that is homozygous
recessive for all three traits? (1/4)3
1/64 How tall would that child be if it were a
girl? 47
54
BbEe x bbee
55
Pea Plants

• In sweet peas, purple flower color (P) is
  dominant over white (p), but there is also a
  control gene such that if the plant has a C,
  the purple has permission to express itself.
• If the plant is cc, the purple does not have
  permission to express itself and the flower will
  be white anyway.
• If a plant with homozygous purple, controlled
  flowers(CC) is crossed with a plant with white,
  non-controlled(cc) flowers, diagram the Punnett
  square for the F1 and F2 generations and
  calculate the phenotype ratios.
• First, what are the genotypes of the parents in
  the first generation?
• PPCC and ppcc
• What are the genotypes of their offspring?
• PpCc

56

• What is always the phenotypic ratio for a
  dihybrid cross

genotype P-C- P-cc ppC- ppcc
9 Dominant for both traits 3 Dominant for
first trait and recessive for second 3 Recessive
for first trait and dominant for second 1
Recessive for both traits
What are the phenotypes? P-C- purple P-cc wh
ite ppC- white Ppcc white