Genetics

Prepared by:MONTALBO, Daniel Justin B.AAPD2F

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.

• The Law of Addition is used to combine probabilities, when there are 2 or more ways to arrive at a given outcome.

• The Law of Multiplication is used to combine probabilities of 2 or more different events that need to occur in combination.

• Mendel’s Law of Independent Assortment says that 2 or more different genes, if found on separate chromosomes, are determined independently of each other.

• Many characteristics have been found to follow patterns of inheritance that are modifications of Mendel’s rules.

Keywords

• genegene• alleleallele• dominantdominant• recessiverecessive• charactercharacter• traittrait• phenotypephenotype• genotypegenotype

• pure breedingpure breeding• hybridhybrid• homozygoushomozygous• heterozygousheterozygous• P (parental P (parental

generation)generation)• FF11 (first filial (first filial

generation)generation)• FF22 (second filial (second filial

generation)generation)• Punnett SquarePunnett Square

• forked lineforked line• polygenespolygenes• contributory contributory

alleleallele• non-non-

contributory contributory alleleallele

• continuous continuous variationvariation

• discontinuous discontinuous variationvariation

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)

Purpleflowers

Whiteflowers

x

F1 Generation (hybrids)

All plants hadpurple flowers

F2 Generation

705 plants 224 plants

self-pollination

MONOHYBRID CROSS inheritance of a single trait

• Alternative versions of genes exist (alleles)– Dominant– Recessive

• Organism inherites 2 alleles– Dominant is fully

expressed– Recessive has no

visible effect

• Law of segregation– 2 alleles for one

character separate and go to different gametes

Allele for purple flowers

Locus for flower-color gene

Homologouspair ofchromosomes

Allele for white flowers

P Generation

F1 Generation

F2 Generation

P p

P p

P p

P

p

PpPP

ppPp

Appearance:Genetic makeup:

Purple flowersPP

White flowerspp

Purple flowersPp

Appearance:Genetic makeup:

Gametes:

Gametes:

F1 sperm

F1 eggs

1/21/2

xTrue-breeding plants have identicalalleles.

Gametes each contain only one allele for the flower-color gene. Every gamete produced by one parent has the same allele.

F1 hybrids have a Pp combination. Purple-flower allele is dominant, all hybrids have purple flowers.

Hybrid plants produce gametes, two alleles segregate: half the gametes receiving the P allele and the other half the p allele.

3 : 1

Punnett square: shows all possible combinations of alleles in offspring from an F1 x F1 (Pp x Pp) cross.

Each square represents an equally probable product of fertilization.

Random combination of the gametesresults in the 3:1 ratio that Mendelobserved in the F2 generation.

Other terms in genetics

• Homozygous – identical alleles, true-breeding

• Heterozygous – different alleles

• Phenotype – physical appearance

• Genotype – genetic make-up

The test-cross

X

Dominant phenotype,unknown genotype:

PP or Pp?

Recessive phenotype,known genotype:

pp

If PP,then all offspring

purple:

If Pp,then 1⁄2 offspring purpleand 1⁄2 offspring white:

p p

P

P

Pp Pp

PpPp

pp pp

PpPpP

p

p p

APPLICATION

An organism displaying the dominant phenotype can eitherbe homozygous or heterozygous for the trait. A test-cross willhelp determine which.

TECHNIQUE

The individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent.

RESULTS

Dihybrid cross• Illustrates the inheritance

of two characters• Produces four phenotypes

in the F2 generation

• Law of Addition - combines probabilities for mutually exclusive events

• The Law of Multiplication - combines probabilities of 2 or more independent events that need to occur together

Law of independent assortment- allele pairs separate independently during the formation of gametes

Forked Line Method

AaBbCc x AaBbCc

Monohybrid cross problems1. 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. 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?A. If all of the offspring of a particular cross have the genotype Yy, what must the genotype of the parents be?B. 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 problems1. 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? 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 white2. 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 problems1. 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 problems5. 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.