genetics - bio resource site...codominance - two dominant alleles affect phenotype in separate ways...
TRANSCRIPT
Genetics
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
Results for other hybrid (F1) characters
Blending
inheritance
Complete
dominance
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
• Produces
four
phenotypes
in the F2
generation
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)
Law of independent
assortment Allele pairs separate independently
during the formation of gametes
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?
Reminders for the Perio
1. Last exam on Thursday (PM sked), after Math
2. Coverage: Cell Cycle – Non-Mendelian inheritance; guide questions available online
3. NBs (4% of grade) + any resubmissions due at 2:30 PM on the day of the Bio Perio
4. EoQRs available online due Aug 11 (Sun), 12 NN
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Find the genotypic
and phenotypic
ratios of the offspring
from the following
crosses using the
forked line method:
1. AaBb x aaBb
2. AABbCc x AaBbcc
Codominance
- two dominant alleles affect phenotype in separate ways
- both alleles manifest
- e.g. roan coloring in horses – both red and white
Incomplete dominance
- phenotype of offspring is between phenotypes of two parents
- e.g. red and white parents give rise to pink offspring
Multiple alleles
- genes with more than two alleles that control the phenotype
- e.g. ABO blood type system has 3 alleles--IA,IB,i. IA,IB are codominant, i is recessive to both
Non-mendelian inheritance Alleles with different degrees of dominance and recessiveness
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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.