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Core Concepts • Mendel’s Law of Segregation states that there are two alleles
for every gene determining a specific characteristic, and these
alleles are segregated into separate gametes during
reproduction.
• When the 2 different alleles occur together in one individual
(heterozygote), the dominant allele will be the one that is
expressed in the phenotype of the individual.
• Mendel’s Law of Independent Assortment says that 2 or
more different genes, if found on separate chromosomes, are
determined independently of each other.
• The Addition Rule is used to combine probabilities, when
there are 2 or more ways to arrive at a given outcome.
• The Multiplication Rule is used to combine probabilities of 2
or more different events that need to occur in combination.
• Many characteristics have been found to follow patterns of
inheritance that are modifications of Mendel’s rules.
Keywords
• gene
• allele
• dominant
• recessive
• character
• trait
• phenotype
• genotype
• pure breeding
• hybrid
• homozygous
• heterozygous
• P (parental generation)
• F1 (first filial generation)
• F2 (second filial generation)
• Punnett
Square
• forked line
Mendel’s
Discoveries • Pre-Mendel beliefs in
genetics: blending theory
• Augustinian monk and science teacher
• Why Pisum sativum? – Pea plants available in many
varieties • character (heritable feature)
• trait (character variant)
– Perfect flowers • cross-pollination and self-
pollination
– Short generation time
– Many offspring
CHARACTERS TRAITS
P Generation (true-breeding parents)
Purple flowers
White flowers
x
F1 Generation (hybrids)
All plants had purple flowers
F2 Generation
705 plants 224 plants
self-pollination
MONOHYBRID CROSS inheritance of a single trait
Cross-
fertilization
Mendelian
Inheritance Complete dominance
• Alternative
versions of genes
exist (alleles)
– Dominant
– Recessive
• Organism inherits
2 alleles
– Dominant is fully
expressed
– Recessive has no
visible effect
Allele for purple flowers ( P )
Locus for flower-color gene Homologous pair of chromosomes
Allele for white flowers ( p )
• Alternative
versions of
genes exist
(alleles) • Dominant
• Recessive
• Organism
inherits 2
alleles
(genotype) • Dominant is
fully expressed
• Recessive has
no visible effect
PP pp
Pp
PP or
Pp pp
Other terms in
genetics
• Homozygous – identical
alleles, true-breeding
• Heterozygous – different
alleles
• Phenotype – physical
appearance
• Genotype – genetic make-up
Mendelian
Inheritance Law of Segregation
2 alleles for one
character
separate and go
to different
gametes
Parental generation
• Pure-breeding parents are homozygous for
different traits for flower color
• Each parent produces only one type of
gamete: purple parent (P), white
parent (p)
F1 generation
• Hybrids are heterozygous for flower color
• Can produce two kinds of gametes: P
or p
F2 generation
• Due to the law of segregation, eggs and
sperm with different kinds of gametes
combine to form a specific ratio
• Genotypic ratio 1 PP : 2 Pp : 1 pp
• Phenotypic ratio 3 purple : 1 white
Problem-solving An organism displays the
dominant phenotype. It
may be either homozygous
or heterozygous for the
trait.
How can its genotype be
determined?
Perform a test-cross:
Dominant phenotype x
recessive phenotype
Dihybrid cross Inheritance of two characters
• Characters are
NOT
transmitted as a
package (YR
and yr only)
• Genes are
packaged into
gametes in all
possible
combinations of
alleles (YR, Yr,
yR, yr)
Genetics and Probability
Addition Rule
combines probabilities for mutually exclusive
events
Multiplication Rule
combines probabilities of 2 or more independent
events that need to occur together
Monohybrid cross Round seed shape is dominant over wrinkled seeds in pea plants.
What percentage of the F2 generation can be expected to:
a. Be homozygous dominant for seed shape?
b. Be heterozygous for seed shape?
c. Have round seeds?
Genetics and Probability
Addition Rule
combines probabilities for mutually exclusive
events
Multiplication Rule
combines probabilities of 2 or more independent
events that need to occur together
Dihybrid cross Round seed shape is dominant over wrinkled seeds in pea plants. Green pod color is dominant over yellow pods.
What percentage of the F2 generation can be expected to:
a. Be heterozygous for both characters?
b. Have round seeds in green pods?
c. Have wrinkled seeds in green pods?
Genetics and
Probability
Addition Rule
combines probabilities for mutually exclusive
events
Multiplication Rule
combines probabilities of 2 or more independent
events that need to occur together
Trihybrid cross Round seed shape is dominant over wrinkled seeds in pea plants. Green pod color is dominant over yellow pods. Tall plants are dominant over short ones.
What percentage of the F2 generation can be expected to:
a. Be heterozygous for all characters?
b. Be tall plants with round seeds in yellow pods?
c. Be short plants with round seeds in green pods?
Monohybrid cross problems 1. In pea plants, spherical seeds (S) are dominant to dented seeds (s). In a genetic cross of
two plants that are heterozygous for the seed shape trait, what fraction of the offspring
should have spherical seeds?
2. In pea plants, yellow seed color is dominant to green seed color.
A. If a heterozygous pea plant is crossed with a plant that is homozygous recessive for seed
color, what is the probability that the offspring will have green seeds?
B. If all of the offspring of a particular cross have the genotype Yy, what must the genotype
of the parents be?
C. To identify the genotype of yellow-seeded pea plants as either homozygous dominant
(YY) or heterozygous (Yy), you could do a test cross with plants of genotype _______.
3. In rabbits, black fur (B) is dominant to white fur (b). Perform the following crosses. For
each cross, give the phenotype and genotype of all offspring.
A. BB x bb B. Bb x Bb C. BB x Bb
4. In summer squash, white fruit color (W) is dominant over yellow fruit color (w). If a
squash plant homozygous for white is crossed with a plant homozygous for yellow, what
will the phenotypic and genotypic ratios be for:
A. the F1 generation? B. the F2 generation?
C. What will the phenotypic and genotypic ratios of the offspring be if you perform a
testcross with the F1 generation?
Dihybrid and trihybrid cross problems 1. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste
certain substances is also genetically controlled. For example, there is a substance
called phenylthiocarbamate (PTC for short), which some people can taste (the
dominant trait), while others cannot (the recessive trait).
A woman who is both a homozygous tongue-roller and a non-PTC-taster
marries a man who is a heterozygous tongue-roller and is a PTC taster, and they
have three children: a homozygous tongue-roller who is also a PTC taster, a
heterozygous tongue-roller who is also a taster, and a heterozygous tongue-roller
who is a non-taster.
A. If these parents have more children so that they had 12 in all, how many of
those 12 would you expect to be non-tasters who are homozygous for tongue-
rolling?
B. If the first child (the homozygous tongue-roller who is also a PTC taster)
marries someone who is heterozygous for both traits, draw the Punnett square
that predicts what their children will be.
2. Using the forked-line method, determine the genotypic and phenotypic ratios of
these trihybrid crosses:
A. AaBbCc x AaBBCC B. AaBBCc x aaBBCc
Non-mendelian inheritance
1. Codominance – two dominant alleles affect
phenotype in separate ways
– both alleles manifest
– e.g. roan coloring in horses – both red and white
2. Incomplete dominance – phenotype of offspring is
between phenotypes of two parents
– e.g. red and white parents give rise to pink offspring
3. Multiple alleles – genes with more than two
alleles that control the phenotype
– e.g. ABO blood type system has 3 alleles--A,B,i. A and B are codominant, i is recessive to both
Sample problems
1. A cross between a blue blahblah bird & a white blahblah bird produces silver blahblah birds.
a) What are the genotypes of the parent blahblah birds in the original cross?
b) What is the genotype of the silver offspring?
c) What would be the phenotypic ratios of offspring produced by two silver blahblah birds?
2. The color of fruit for plant "X" is determined by two alleles. When two plants with orange fruits are crossed the following phenotypic ratios are present in the offspring: 25% red fruit, 50% orange fruit, 25% yellow fruit. What are the genotypes of the parent orange-fruited plants?
3. Predict the phenotypic ratios of offspring when a homozygous white cow is crossed with a red roan bull.
4. What should the genotypes & phenotypes for parent cattle be if a farmer wanted only cattle with red fur?
Sample problems
5. A cross between a black cat & a tan cat produces a tabby pattern (black & tan fur together).
a) What pattern of inheritance does this illustrate?
b) What percent of kittens would have tan fur if a tabby cat is crossed with a black cat?
6. A woman with Type O blood and a man who is Type AB are expecting a child. What are the possible blood types of the kid?
7. What are the possible blood types of a child whose parents are both heterozygous for "B" blood type?
8. What are the chances of a woman with Type AB and a man with Type A having a child with Type O?
9. Determine the possible genotypes & phenotypes with respect to blood type of the offspring of a couple whose blood types are A and B. Both sets of grandparents have the AB blood type.