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Chapter 12:
Mendel and
Heredity
Heredity – The passing of traits from
parents to offspring
Genetics – The study of heredity
Gregor Mendel
• Austrian monk
• Bred pea plants
• 1860 - developed laws
of heredity
mms://204.13.204.36/Video9/mendelslaw.asf
•He cross-pollinated plants
•He bred plants to be pure for certain traits
•Ex: Tall parent  tall offspring
Short parent  short offspring
•Then he cross-bred plants with opposite
traits
•Tall x short
•Round x wrinkled
•Yellow x green
•Parents - P generation – tall x short
•Offspring – F1 generation (1st filial
generation)
•All offspring were tall (short trait
disappeared)
•Allowed F1 generation to self-pollinate
•F2 generation – 75% tall to 25% short
(short trait reappeared)
•Repeated many times – always same ratios
for each generation (see results slide #2)
Mendel’s Conclusions:
•There are 2 factors for every trait (today
we know these factors to be genes – 1
from mother, 1 from father)
•One of these factors can be dominant
over the other (the recessive trait)
•This is known as the Law of
Dominance
• Alleles – each version of a gene
• Ex: tall and short height
curly and straight hair
brown and blue eyes
What is the relationship between
chromosomes, DNA, genes, and alleles?
•Chromosomes contain DNA and protein
•DNA contains genes – each gene always has the same
position on a chromosome (locus)
•Alleles are different forms of the same gene (right vs.
left-handed, blue vs. brown eyes)
Widow’s peak
•When an organism produces gametes, each
pair of alleles is separated and each gamete
has an equal chance of receiving either one of
the alleles –The Law of Segregation
•Each gamete only has 1 factor from each pair
(haploid)
•Fertilization gives each new individual 2
factors again (diploid)
•Mendel then crossed pure plants that
differed in 2 traits
•Ex: yellow, round peas crossed with
green, wrinkled peas
•F1 generation always showed dominant
traits
•F2 generation had the following results:
(see next slide)
F2: 9 yellow, round
3 yellow, wrinkled
3 green, round
1 green, wrinkled
•Based on these results, Mendel concluded that
during gamete formation, the alleles of each gene
segregate independently – The Law of Independent
Assortment
•Ex: Below, hairline and finger length are not
dependent on each other
•Mendel’s Law of Independent
Assortment would not hold true if the
genes for two traits are located on the
same chromosome – linked genes
Genotype and Phenotype
• Genotype refers to the set of alleles that an
individual has for a character; can be
represented by two letters
• Homozygous - both alleles are the same
• Homozygous dominant - WW
• Homozygous recessive - ww
• Heterozygous – alleles are different - Ww
• Phenotype refers to the trait that results
from a set of alleles.
• Both WW and Ww result in widow’s
peak, the dominant trait
• ww will result in no widow’s peak, the
recessive trait
Monohybrid Crosses
• Considers only one trait.
• Punnett square – chart that shows all of
the genotypes that could result from a
given cross
•Ratio shows #
of offspring
with dominant
vs. recessive
trait
Probability
• The likelihood that a specific event will occur.
• Expressed as fraction or percentage
• Ex: (1/4) or 25%
• The probability that two or more independent
events will occur together is the product of
their chances occurring separately
• Ex: odds of having a boy = ½
Odds of having 2 boys = (1/2) x (1/2) =
(1/4)
• The chance of widow’s peak:
• WW or Ww = 75% or ¾
• Chance of a continuous hairline:
• ww = 25% or 1/4
•Odds of
having 3
children with a
continuous
hairline:
•(1/4) x
(1/4) x (1/4)
= (1/64)
Dihybrid Cross
• Two traits are considered
• Genotypes of the parents require four letters
(two for each trait).
Multiple Allelic Traits
• More than two alternative alleles exist for
a particular trait.
• Ex: blood type – A, B, and O are the 3
alleles
• Each individual inherits only two alleles
for these genes.
• Codominance - both alleles for the same
gene are fully expressed in a heterozygote
• Ex: Blood type – AB blood
• Incomplete dominance – An offspring has
a phenotype that is intermediate between
the traits of its parents.
• Ex: Curly, wavy, or straight hair in
Caucasians
ABO Blood Types
• How your book shows blood type:
Blood type
(phenotype)
A
Genotype
IAIA or IA i
B
IBIB or IBi
AB
IA IB
O
ii
•How your teacher shows blood type:
Blood type
(phenotype)
A
Genotype
AA or AO
B
BB or BO
AB
AB
O
OO
Blood type
Donor for
A
A, AB
Recipient
from
A, O
B
B, AB
B, O
AB
AB
A, B, AB, O
O
A, B, AB, O
O
Inheritance of blood type
Incomplete dominance
• Other examples of incomplete dominance:
• Plants called four o’clocks
RR – red
RR’ – pink
R’R’ – white
R
R
RR
• So a cross
RR’
R’
between two
pink plants produces
1 red, 2 pink, and 1 white plant
R’
RR’
R’R’
• Another example includes Sickle cell disease
in humans
• HbA represents normal hemoglobin; and HbS
represents the sickled condition
– HbAHbA – normal
– HbSHbS – sickle-cell disease
– HbAHbS - have the intermediate condition
called sickle-cell trait.
• Heterozygotes have an advantage in malariainfested Africa because the pathogen for
malaria cannot exist in their blood cells.
Sex determination:
•Female – XX
•Male – XY
•Always 50%
X
chance of having a
boy or a girl
Y
•Male determines
gender of baby
X
X
XX
XX
XY
XY
Sex-Linked Traits
• Traits controlled by genes on the X or Y
chromosomes
• X-linked or Y-linked
• Most X-linked traits are recessive, so a
female would have to have two recessive
genes to express the trait; a male would
only need one.
• Y-linked traits are only passed from
father to son
• Examples of X-linked traits include Color
blindness, Hemophilia, & Muscular
Dystrophy
Phenotype
Normal female
Carrier female
Affected
female
Normal male
Affected male
Gentoype
XBXB
XBXb
XbXb
XBY
XbY
Cross involving an X-linked allele
Sex Influenced Traits
• Sex-influenced traits are autosomal traits that
are influenced by sex. If a male has one
recessive allele, he will show that trait, but it
will take two recessive for the female to show
that same trait. One such gene is baldness.
• BB normal male & female
• Bb  bald male; normal female
• bb  bald male; bald female
Pedigree Charts
• Constructed to show the pattern of
inheritance of a characteristic within a
family.
• The particular pattern indicates the
manner in which a characteristic is
inherited (suggests X-linked, dominant,
etc.)
Symbols used:
Normal female
Carrier female
Affected female
Normal male
Affected male
Male can be a carrier for an autosomal
trait.
Autosomal recessive pedigree chart
Autosomal dominant pedigree chart