Genetics Core Concepts Mendels Law of Segregation states that there - - PowerPoint PPT Presentation

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
• r 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 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 Pp PP PP or Pp 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
• r 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

• r 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

• ccur together

Monohybrid cross

Round seed shape is dominant

• ver 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

• ccur together

Dihybrid cross

Round seed shape is dominant

• ver 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

• ccur together

Trihybrid cross

Round seed shape is dominant

• ver 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?

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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
• f the parents be?
• C. T
• 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

• f grandparents have the AB blood type.