Genetics II: Mendelian Genetics
I. Gregor Mendel- (1822-1884) the Father of Modern Genetics
A. History and Early Studies
1. at 21 he entered monastery, at 25 became a priest
2. 1851 went to the University of Vienna for math and science for 2 yrs,
taught HS for 14 yrs
3. he knew pea plants reproduced sexually by self-pollination
4. he knew there were 2 types of gametes/sex cells
(male= pollen, female= ovule)
5. learned that self-pollination in pea plants could be prevented if male
gametes or female gametes were taken away to stop fertilization.
6. cross-pollination- taking male part of one pea plant and female part
of another pea plant and using them to fertilize each other.
B. Later Studies (published work in 1866)
Dominant
phenotype
1. Mendelian Genetics- he looked at clear, distinct
traits in pea plants (see table to the right)
2. He noticed that there seemed to be 2 distinct
physical expressions for each trait
3. Capital letters signify dominant traits and lower case
are recessive ex. Stem (tall = T, short = t)
Seed coat (gray = G, white = g)
C. Mendel’s Conclusions or Principles
1. Dominance & Recessiveness- one factor (gene) in a
pair may mask the other factor (gene), preventing it
from having an effect.
2. Law of Segregation- 2 alleles for each trait separate
when gametes form.
3. Law of Independent Assortment- traits are inherited
independently if they’re on different xsomes.
Graphing Dominant Human Traits: widow’s peak, gap in teeth,
taster, free ear lobes, mid-digit hair, dimples, freckles, hitchhikers
thumb, thumb fold right.
II. Terms
heredity- **
genetics- **
traits- **
genome- **
alleles- different forms of the same gene, get 2 alleles for each trait (mom & dad) tall/short
homozygous- two alleles of ** form (2 dominant or 2 recessive)
heterozygous- ** alleles (one dominant and one recessive)
recessive- form of gene only expressed in homozygous state, masked by dominant gene
dominant- form of a gene that always shows even if recessive allele is present
phenotype- organism's ** caused by expression of genes
genotype- type of gene alleles present in organism's genome (two letters for each trait)
test cross- genotypically homozygous recessive organism crossed with a phenotypically dominant
organism, used to determine the genotype of an unknown
Recessive
phenotype
III. Punnett Squares
A. Monohybrid Crosscross between 2 parents that differ by only 1 trait
Mendel began cross pollinating purebred pea
plants to make hybrids
tips: the dominant allele always goes first (Tt)
male gametes go along the top,
female gametes go along the side,
offspring inside
1. Law of Dominance & Recessivenessone factor (gene) in a pair may mask the
other factor (gene), preventing it from
having an effect.
2. Law of Segregation- 2 alleles for each trait
separate when gametes form.
3. hybrid- crossing 2 characteristics (tall vs short)
4. monohybrid cross- cross btwn 2 parents (P) that differ by only 1 trait
ex. cross 2 plants that are identical for every trait except height (TT x tt)
ALL of the offspring in
the first generation (F1)
were phenotypically tall
5. to Mendel, it looked as if the short trait "disappeared"
6. he crossed 2 of the F1 offspring and the short “reappeared” 1 out of 4 times
the phenotypes of the offspring in the 2nd
generation (F2) showed a 3:1 phenotypic ratio
7. Mendel concluded that each trait has 2 factors (genes) for each trait passed down
8. each form of a gene is called an ** (tall vs short)
9. one allele is contributed by each parent, 1 allele on each homologous chromosome
10. in Mendelian genetics, for each trait there is one allele that dominates the other (tall over short)
a. 2 of the same alleles = ** (recessive or dominant)
b. 2 different alleles = ** (one dominant, one recessive)
c. ** - combination of alleles, type of genes
d. ** - physical appearance of an organism
B. Dihybrid Cross- cross between 2 parents that differ by only 2 traits
tips: the dominant allele always goes first (TtGg...TtGG)
alleles of traits are kept together and in the same order throughout (TTGg...ttGg)
tips for punnett squares:
a. determine genotypes of parents
b. determine gametes
c. do the punnett square
1. Mendel crossed height (Tall vs short) and pod color (Green vs yellow)
ex. cross 2 plants homozygous tall, green and homozygous short, yellow
2. results- all F1 generation were tall and green
the phenotypes of the
offspring in the 1st
generation (F1) showed a
4:0 phenotypic ratio
3. Mendel crossed the F1 offspring
4. results- 9: 3: 3: 1 (9 tall,green: 3 tall,yellow: 3 short,green: 1 short,yellow)
the phenotypes of the offspring in
the 2nd generation (F2) showed a
9:3:3:1 phenotypic ratio
5. Law of Independent Assortment- different traits are inherited independently if they’re on
different xsomes.
Can a pale blue person (aabb) and a violet blue eyed person (aaBb) have a green eyed child?
IV. Walter Sutton (1902) Developed the Chromosome Theory of Heredity
A. looked at Mendel’s findings and said the "factors" Mendel spoke of were XSOMES
B. xsomes did the major things that Mendel saw in his work
1. they occur in pairs
2. they segregate during gamete formation (MEIOSIS)
3. pairs of alleles assort independently if they are not linked on same xsome.
V. Patterns of Heredity
A. Sex-linked traits aka. X-linked traits
1. controlled by genes on ** (23rd pair of xsomes)
2. fly eye color, colorblindness in men, hemophilia, feathers in birds
Phenotypes
Female
Genotypes
Male
Genotypes
Normal
Carrier
Affected
XBXB
XBXb
XbXb
XBY
none
XbY
B. Incomplete dominance
1. traits are blended; phenotype of a heterozygote is btwn homozygous dominant & recessive
2. pure breed red flower [RR] crossed w/ pure breed white flower [rr] = pink [Rr]
C. Codominance
1. both alleles are expressed **
2. black rooster [BB], white hen [ bb] = checkered (spotted) [Bb]
D. Multiple Alleles
1. blood typing- controlled by multiple alleles (A, B, O) that are codominant
2. phenotype or blood types are A, B, O and AB
3. universal donor= O-, universal acceptor= AB-, rare= AB+
Blood Types
Phenotypes
A
B
AB
O
Genotypes
IAIA or IAi
IBIB or IBi
IAIB
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