Mendelian Genetics

1
Mendelian Genetics

• Based on the PowerPoint Presentations from
  Campbell Biology (AP), 8th Ed.

2
The Particulate Hypothesis

• Many scientists that came before Mendel thought
  that traits were blended.
• We now know that this is not upheld by evidence.
• Others like Mendel thought there were discrete
  units of inheritance (what we now know as genes).
• These scientists turned out to be correct.

3
Why Pea Plants?

• Mendel studied heredity in pea plants because
• They have distinct characters (either/or)
• Each character has variations (ex wrinkled or
  round peas)
• Their mating can be controlled
• Each pea flower has both male and female parts
  (can accomplish self-fertilization)
• Cross-pollination is also possible

4
Hybridization
Fig. 14-3-3
EXPERIMENT
P Generation (true-breeding parents)
?

• A cross between two different true breeding
  plants (plants that always produce offspring
  identical to themselves when they self-pollinate)
• P-generation true-breeding parents.
• F1 generation hybrid offspring of P.
• F2 generation offspring of F1 due to
  self-pollination.

Purple flowers
White flowers
F1 Generation (hybrids)
All plants had purple flowers
F2 Generation
224 white-flowered plants
705 purple-flowered plants
5
Mendels Model

• The F2 generation always showed a 31 ratio
  between the two trait variants.
• Mendel named the more common trait the dominant,
  and the less common the recessive.
• We now call these variants alleles.
• We also know they are located at distinct loci
  (locations) on chromosomes.

Allele for purple flowers
Homologous pair of chromosomes
Locus for flower-color gene
Fig. 14-4
Allele for white flowers
6
Mendels Model

• With no knowledge of chromosomes, Mendel
  concluded that each individual inherits 2 alleles
  for each trait.
• One from each parent
• Law of Segregation- the two alleles of the parent
  are split into separate gametes during their
  formation (meiosis).
• True-breeding 2 identical alleles (homozygous)
• Hybrid 2 different alleles (heterozygous)
• If hybrid, then the dominant allele determines
  the characteristic (Law of Dominance).

7
Punnett Squares
P Generation
Purple flowers
Appearance
White flowers
Genetic makeup
PP
pp
p
Gametes
P

• Can be used to determine the possible outcome of
  a cross between two individuals

F1 Generation
Appearance
Purple flowers
Genetic makeup
Pp
p
Gametes
1/2
1/2
P
Sperm
F2 Generation
p
P
P
PP
Pp
Eggs
p
Pp
pp
Fig. 14-5-3
3
1
8
Traits do not always reveal genes.
Fig. 14-7
TECHNIQUE
?
Dominant phenotype, unknown genotype PP or Pp?
Recessive phenotype, known genotype
pp

• Phenotype- an organisms appearance (physical
  traits).
• Genotype- an organisms genes.
• By looking at an organism with dominant
  characters, there is no way to tell if they are
  homozygous or heterozygous.
• A test-cross can be done to determine this.

Predictions
If PP
If Pp
or
Sperm
Sperm
p
p
p
p
P
P
Pp
Pp
Pp
Pp
Eggs
Eggs
P
p
pp
Pp
Pp
pp
RESULTS
or
All offspring purple
1/2 offspring purple and 1/2 offspring white
9
Law of Independent Assortment
Fig. 14-8a
EXPERIMENT
YYRR
yyrr
P Generation
Gametes
yr
YR
?
F1 Generation

• Mendel followed two characters at a time.
• He determined that traits are not inherited
  together, but separately.
• We no know this law only applies to traits on
  non-homologous chromosomes because they are
  unlinked.
• Genes near each other will likely be inherited
  together.

YyRr
Hypothesis of independent assortment
Hypothesis of dependent assortment
Predictions
Sperm
or
Predicted offspring of F2 generation
yr
Yr
1/4
1/4
1/4
YR
yR
1/4
Sperm
yr
YR
1/2
1/2
1/4
YR
YYRR
YyRr
YYRr
YyRR
YR
1/2
YYRR
YyRr
Yr
1/4
Eggs
YyRr
Yyrr
YYRr
YYrr
Eggs
yr
1/2
yyrr
YyRr
yR
1/4
YyRr
YyRR
yyRR
yyRr
1/4
3/4
yr
1/4
Phenotypic ratio 31
YyRr
yyRr
Yyrr
yyrr
9/16
3/16
3/16
1/16
Phenotypic ratio 9331
10
Rules of Probability

• The probability of inheriting a Mendelian trait
  can be calculated by simple mathematic laws
• The probability of independent events occurring
  together are multiplied.
• The probability of exclusive events occurring
  together are added.

Fig. 14-UN1
11
Mendels Laws apply mostly to traits controlled
by a single gene.

• Inheritance of characters by a single gene may
  deviate from simple Mendelian patterns in the
  following situations
• When alleles are not completely dominant or
  recessive
• When a gene has more than two alleles
• When a gene produces multiple phenotypes

12
Degrees of Dominance

• Complete Dominance
• The phenotype of the heterozygote is identical to
  the dominant.
• Ex In pea plants, both YY and Yy yellow peas.
• Incomplete Dominance
• Hybrids show an intermediate phenotype between
  the dominant and recessive.
• Ex In some flowers RR red, Rr pink, rr
  white.
• Codominance
• Two dominant alleles affect the same trait, but
  in different ways.
• Ex In human blood type, AB is the codominance of
  both the A and B alleles.

13
Dominance

• Just because a trait is dominant does not always
  mean its more prevalent.
• Animals can choose traits by choosing mates.
• Ex Having 6 fingers and toes is a dominant
  trait, although very few people are actually born
  this way.

14
Non-Mendelian Traits

• Pleiotropy- A gene has multiple phenotypic
  effects.
• Polygenic inheritance- Two or more genes affect a
  single trait
• These traits tend to vary over a continuum and
  are called quantitative characters.
• Ex Epistasis, a gene at one locus alters the
  phenotypic expression of a gene at a second locus

15
Nature vs. Nurture

• The norm of reaction is the phenotypic range of
  a genotype influenced by the environment.
• Polygenic characters are often influenced more by
  environment than single-gene traits.
• Multifactorial traits- affected by many genes and
  the environment.

16
Pedigree

• Used to trace traits through a family.
• Universal key for pedigrees
• Pedigrees can also be used to predict the traits
  of future generations.

Fig. 14-15a
Key
Mating
Male
Affected male
Offspring, in birth order (first-born on left)
Female
Affected female
17
Fig. 14-15b
1st generation (grandparents)
Ww
Ww
ww
ww
2nd generation (parents, aunts, and uncles)
Ww
Ww
ww
ww
ww
Ww
3rd generation (two sisters)
ww
WW
or
Ww
Widows peak
No widows peak
(a) Is a widows peak a dominant or recessive
trait?
18
Recessive Disorders

• Ex Albinism, Cystic Fibrosis, Sickle-Cell Anemia
• Only shown in individuals with no dominant
  allele.
• Heterozygotes show the normal phenotype, but
  are carriers and can pass on the recessive
  allele.
• If a disease is rare, then matings between
  carriers should also be rare.
• Consanguineous matings (matings between close
  relatives) increase the chance of mating between
  two carriers of the same rare allele

19
Dominant Disorders

• Ex Achondroplasia (dwarfism), Huntingtons
  disease
• These diseases are more rare because there are no
  carriers (homozygous recessives are the only ones
  without the disease- but they tend to increase in
  number as the disease is selected against and/or
  kills of those who have it).

20
Testing for Genetic Diseases

• Amniocentesis
• A sample of amniotic fluid (surrounding the
  fetus) is taken and chromosomes are analyzed for
  abnormalities (takes several weeks).
• Chorionic villus sampling (CVS)
• A placenta sample is taken and analyzed (takes
  several hours).
• Ultrasound
• The fetus is viewed in utero
• Other tests can be done at birth to test for
  certain diseases and disorders.

21
You should now be able to

• Define the following terms true breeding,
  hybridization, monohybrid cross, P generation, F1
  generation, F2 generation
• Distinguish between the following pairs of terms
  dominant and recessive heterozygous and
  homozygous genotype and phenotype
• Use a Punnett square to predict the results of a
  cross and to state the phenotypic and genotypic
  ratios of the F2 generation

• Explain how phenotypic expression in the
  heterozygote differs with complete dominance,
  incomplete dominance, and codominance
• Define and give examples of pleiotropy and
  epistasis
• Explain why lethal dominant genes are much rarer
  than lethal recessive genes
• Explain how carrier recognition, fetal testing,
  and newborn screening can be used in genetic
  screening and counseling