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HOW INHERITED TRAITS
ARE TRANSMITTED
Chapter 12
Genetics is the science of heredity.
Gregor Mendel
 Austrian
monk with a strong
mathematical background.
 Interested in how certain traits were
passed from parents to offspring.
 Worked with pea plants (1857-1863):
 easy to grow & developed quickly
 exhibited many traits that had 2
easily distinguishable forms
 could manipulate fertilizations
Traits Mendel Studied
Mendel’s Experimental Approach for
Breeding Peas
Mendel’s observations:
 Some
pea plants were always “true
breeding” (all offspring exhibit same trait
as parents).
short x short  all short offspring
tall x tall  all tall offspring OR
 some tall, some short
 One form of a trait tended to “mask”
expression of the other form.
tall x short  all tall offspring OR
 some tall, some short
A tall x short mating never resulted in
all short offspring.
Thus, the tall trait masked expression
of the short trait.
Based upon these & subsequent
observations, Mendel formulated
the two basic laws of heredity:
 Law
of Segregation
 Law of Independent Assortment
A. Genetic Terminology
1. Chromosome - dark staining body in a
cell’s nucleus; consists of DNA &
proteins.
 haploid cells - 1 set of chromosomes
 diploid cells - 2 sets of chromosomes
2. Gene (elementen) - a sequence of
DNA that codes for production of a
specific protein.
Ex. stem length gene, seed color gene
3. Allele - an alternate form of a gene.
Ex. stem length gene has 2 alleles - tall
allele & short allele
 Dominant
allele - allele that masks the
expression of another allele.
Ex. tall allele (T)
 Recessive
allele - allele whose
expression is masked by another allele.
Ex. short allele (t)
Diploid organisms possess 2 alleles for
each gene.
2 dominant alleles for gene ‘A’
1 dominant & 1 recessive allele
for gene ‘B’
2 recessive alleles for gene ‘D’
If the 2 alleles are identical, then
organism is homozygous for that gene.
Ex. TT (homozygous dominant)
tt (homozygous recessive)
If the 2 alleles are different, then
organism is heterozygous for that
gene.
Ex. Tt
4. Genotype - the allele combination in an
individual.
Ex. three possible genotypes for pea plant
height: TT, Tt or tt
5. Phenotype - the observable expression
of an allele combination.
Ex. two possible phenotypes for pea plant
height: tall or short
If you know an individual’s
phenotype, do you automatically
know their genotype?
 Yes,
if the trait is recessive
 short pea plants must be tt
 round-seeded plants must be rr
 No, if the trait is dominant
 tall pea plants can be either TT or Tt
 wrinkled-seeded plants can be either
RR or Rr
B. Law of Segregation
The alleles of a gene separate during
meiosis as chromosomes are
packaged into gametes.
We use a Punnett square to predict
the outcome of a cross between
two individuals.
Phenotypic ratio
3 tall : 1 short
Genotypic ratio
1 TT : 2 Tt : 1 tt
We use a test cross to determine an
unknown genotype.
Test cross = a cross between an
individual of unknown genotype and an
individual that is homozygous
recessive for the trait in question.
You are given a tall pea plant…
determine it’s genotype.
Do a test cross: tt x unknown
 if
tt
TT
all offspring are tall, THEN…
unknown genotype is TT
if
obtain both tall & short
offspring, THEN…
unknown genotype is Tt
Tt
tt
Tt
Tt tt
Mendelian Disorders in Humans:
Autosomal Recessive Traits:
 located on non-sex chromosomes
 parents are carriers or are affected
 affected individuals are homozygous
recessive
 affects males & females
Ex. Albinism, Cystic fibrosis,
Phenylketonuria, Sickle cell disease
Autosomal Dominant Traits:
 located on non-sex chromosomes
 at least one parent is affected
 does not skip generations
 affected individuals are homozygous
dominant or heterozygous
 affects males & females
Ex. Achondroplasia, Huntington disease,
Lactose intolerance, Polydactyly
Autosomal dominant
pedigree
C. Law of Independent Assortment
The segregation of one gene pair does
not influence the segregation of
another gene pair during meiosis.
D. Factors Appearing to Violate
Mendel’s Laws
1. Lethal Alleles - certain allele
combination causes death of an entire
phenotypic class very early in
development.
Ex. hairless trait in dogs [homozygous
dominant (HH) individuals die as embryos]
2. Multiple Alleles - gene exists as more
than two alleles in the population.
 Rabbit coat color gene has 4 alleles:
C, c, cch & ch
•5 phenotypes
•10 genotypes
3. Epistasis -
one gene masks
the expression
of another.
Ex. Bombay
phenotype
(H gene masks
expression of I
gene)
4. Incomplete Dominance
- heterozygote expresses a
phenotype intermediate
between those of the two
homozygotes.
Ex. snapdragon flower color
If cross Rr x Rr …
Phenotypic ratio ->
1 red : 2 pink : 1 white
Genotypic ratio ->
1 RR : 2 Rr : 1 rr
5. Codominance - heterozygote
expresses a phenotype that is distinct
from and not intermediate between
those of the two homozygotes.
Ex. Human AB blood type
E. Complex Traits
Traits that do not follow Mendel’s
laws, but tend to “run in families”.
1. Polygenic Traits - determined by the
combined effect of more than one
gene.
Ex. height, eye color & skin color
Human Skin Color
2. Multifactorial Traits - determined
by the combined effect of one or more
genes plus the environment.
Ex. heart disease, body weight,
intelligence