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Mendel & The Gene Idea
Chapter 14
Mendelian Genetics
■ Character

heritable feature; ex - fur color
■ Trait (allele)

variant for a character; ex- brown
■ True-bred (purebred)

homozygous for a trait
■ Hybridization

crossing of 2 different purebreds; ex – purple
with white
■ P generation

parents
■ F1 generation

first filial generation; filial = “son”; product of
P generation
■ F2 generation

product of crossing two from the F1 generation
Genetics vocabulary…….
■ Punnett square: predicts the results
of a genetic cross between
individuals of a known genotype
■ Homozygous: pair of identical
alleles for a character
■ Heterozygous: two different
alleles for a character
■ Phenotype: an organism’s
physical appearance
■ Genotype: an organism’s genetic
makeup (allele combination)
■ Testcross: breeding of a recessive
homozygote with a dominate
phenotype (but unknown genotype)
The 3:1 Ratio: What Must Be True
1. Alternative versions of genes (alleles) account
for variations in inherited characters
2. For each character, an organism inherits 2
alleles, one from each parent
3. If the two alleles differ, then one, the dominant
allele, is fully expressed in the organism’s
appearance; the other, the recessive allele, has
no noticeable effect on the organism’s
appearance
4. The alleles for each character segregate
(separate) during gamete production (meiosis)
ending up in different gametes = Mendel’s
Law of Segregation
Mendel’s Law of Independent
Assortment
■ Mendel derived the Law of Segregation from following a single
allele at a time (monohybrid crosses)
■ Mendel’s second law came about when he was following two
alleles at a time (dihybrid crosses)
■ The segregation of one character does not influence the
segregation of another character during gamete formation =
Mendel’s Law of Independent Assortment
Practice
1. A pea plant heterozygous for inflated pods (Ii) is
crossed with a plant homozygous for constricted pods
(ii). Draw a punnett square for this cross and
determine the genotypic and phenotypic ratios.
2. Pea plants heterozygous for flower position (AaTt)
are allowed to self-pollinate, and 400 of the
resulting seeds were planted. Draw a punnett
square for this cross. How many offspring would be
predicted to have terminal flowers and be dwarf?
See table 14.1, pg 254 (7th ed)or 265 (8th ed)
Probability: Multiplication Rule
■ What is the probability
that the offspring will be
homozygous recessive
(rr)?
■ Multiply the probability
of the egg having an ‘r’
by the probability of the
sperm having an ‘r’
½x½=¼
Probability: Addition Rule
■ What is the probability the
offspring will be
heterozygous?
■ There are 2 ways to be
heterozygous: Rr or rR.
■ Calculate the probabilities
of getting Rr and rR and
then add them together
¼+¼=½
Example Using Probabilities
■ What fraction of offspring from the cross below
would be predicted to exhibit the dominant
phenotypes for both of the characters?
YyRr x Yyrr
Practice
1. For any gene with a dominant allele C and a
recessive allele c, what proportions of the offspring
from a CC x Cc cross are expected to be
homozygous dominant, homozygous recessive, and
heterozygous?
2. An organism with the genotype BbDD is mated to one
with the genotype BBDd. Assuming independent
assortment of these two genes, write the genotypes
of all possible offspring from this cross and use the
rules of probability to calculate the chance of each
genotype occuring.
3. Three characters (flower color, seed color, and pod
shape) are considered in a cross between two pea
plants (PpYyIi x ppYyii). What fraction of offspring
would be predicted to be homozygous recessive for
at least two of the three characters?
Non-Dominant Variants
■ Incomplete dominance: blending
appearance between the
phenotypes of 2 alleles.
Ex: snapdragons
■ Codominance: two alleles that
affect the phenotype in separate,
distinguishable ways.
Ex: sickle cell anemia
■ Multiple alleles: more than 2
possible alleles for a gene.
Ex: human blood types
■ Pleiotropy: genes with multiple
phenotypic effects. Ex: cystic fibrosis
& sickle cell
Genetics for Two or More Genes
■ Epistasis: a gene at one locus
(chromosomal location)
affects the phenotypic
expression of a gene at a
second locus. Ex: mice coat
color
■ Polygenic Inheritance: an
additive effect of two or
more genes on a single
phenotypic character
Ex: human skin pigmentation
and height
Human disorders
■ Can be traced using a
family pedigree
■ Recessive disorders:
 Cystic fibrosis
 Tay-Sachs
 Sickle-cell
■ Dominant disorders:
 Huntington’s
■ Testing:
 Amniocentesis
 chorionic villus sampling (CVS)
Fig. 14-15b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ww
ww
Ww
ww
Ww ww ww Ww
Ww
ww
3rd generation
(two sisters)
WW
or
Ww
Widow’s peak
ww
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?
Fig. 14-15c
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
Ff
FF or Ff ff
Ff
ff
ff
Ff
Ff
Ff
ff
ff
FF
or
Ff
3rd generation
(two sisters)
Attached earlobe
(b) Is an attached earlobe a dominant or recessive trait?
Free earlobe