FUNDAMENTALS OF GENETICSChapter 9

MENDEL’S LEGACY

Mendel observed seven characteristics of pea plants Ex: flower color Each characteristic occurred in two

contrasting traits Trait: genetically determined variant of a

characteristic Ex: yellow flower color

MENDEL’S LEGACY

The seven characteristics: Plant height (long and short) Flower position along stem (axial and

terminal) Pod color (green and yellow) Pod appearance (inflated and constricted) Seed texture (round and wrinkled) Seed color (yellow and green0 Flower color (purple and white)

MENDEL’S EXPERIMENTS

He initially studied each characteristic and its contrasting traits individually Began growing true-breeding plants

True-breeding: pure; always produced offspring with that trait when they self-pollinate

Produced true-breeding plants by self-pollinating pea plants until he had 14

MENDEL’S EXPERIMENTS

He then cross-pollinated pairs of plants that were true-breeding for contrasting traits of a single characteristics Ex: he crossed a plant with purple flowers

and a plant with white flowers This was called the P (parent) generation

MENDEL’S EXPERIMENTS

When the plants matured, he recorded the number of each type of offspring produced by each cross Called the offspring the F1 generation

MENDEL’S EXPERIMENTS

He then allowed the F1 generation to self-pollinate, and the next offspring generation was called the F2 generation He performed hundreds of crosses and

documented every result

MENDEL’S RESULTS AND CONCLUSIONS In one of his experiments, Mendel

crossed a plant true-breeding for green pods with a plant that was true-breeding for yellow pods The F1 generation had all green pods

He then let the F1 generation self-pollinate The F2 generation had ¾ green pods and

¼ yellow pods

MENDEL’S RESULTS AND CONCLUSIONS These results made Mendel believe that

each characteristic is controlled by factors A pair of factors must control each trait

MENDEL’S RESULTS AND CONCLUSIONS Mendel got these results through the

thousand of crosses F1 generation: one trait disappeared F2 generation: trait reappeared in a 3:1

ratio

MENDEL’S RESULTS AND CONCLUSIONS Mendel hypothesized that the trait

appearing in the F1 generation was controlled by a dominant factor because it masked the other trait He thought that the trait that did not

appear in the F1 generation but reappeared in the F2 generation was controlled by a recessive factor

SUPPORT FOR MENDEL’S CONCLUSIONS Most of Mendel’s findings agree with

what biologists now know about molecular genetics Molecular genetics: the study of the

structure and function of chromosomes and genes

SUPPORT FOR MENDEL’S CONCLUSIONS Allele: each of two or more alternative

forms of a gene Mendel’s factors are now called alleles

Letters are used to represent alleles Dominant alleles: represented by a capital

letter Recessive alleles: represented by a

lowercase letter

GENOTYPE AND PHENOTYPE

Genotype: an organism’s genetic makeup Consists of the alleles that the organism

inherits from its parents Ex: flower color

Purple flowers: either PP or Pp White flowers: pp

P is the dominant allele P is the recessive allele

GENOTYPE AND PHENOTYPE

Phenotype: an organism’s appearance Since PP and Pp are dominant genotypes,

they will have purple flowers Since pp is a recessive genotype, they will

have the recessive phenotype, which is white flowers

GENOTYPE AND PHENOTYPE

Homozygous: when both alleles of a pair are alike An organism may be homozygous

dominant or homozygous recessive Ex: PP or pp

Heterozygous: when the two alleles in the pair are different Ex: Pp

PROBABILITY

Probability: the likelihood that a specific event will occur May be expressed as a decimal, a

percentage, or a fraction Determined by the following equation:

P= # of times an event is expected to happen # of times an event could happen

PROBABILITY

For example, the dominant trait of yellow seed color appeared in the F2 generation 6,022 times

The recessive trait of green seed color appeared 2,001 times The total number of individuals was 8,023

PROBABILITY

Probability that the dominant trait will appear:

6,022------------ = 0.75 or 75% or ¾ or

3:18,023

PROBABILITY

The results predicted by probability are more likely to occur when there are many traits

PROBABILITY

Probability that the recessive trait will appear:

2,001----------- = 0.25or 25%or ¼ or

1:38,023

PREDICTING RESULTS OF MONOHYBRID CROSSES

Monohybrid cross: a cross in which only one characteristic is tracked The offspring are called monohybrids

Biologists use a Punnett square to aid them in predicting the probable distribution of inherited traits in the offspring

EXAMPLE 1: HOMOZYGOUS X HOMOZYGOUS

PP and pp All offspring are

Pp 100% probability

that the offspring will have the genotype Pp and thus the phenotype purple flower color

EXAMPLE 2: HOMOZYGOUS AND HETEROZYGOUS

BB and Bb Offspring are BB and

Bb The probability of an

offspring having BB genotype is 2/4 or 50%

The probability of an offspring having Bb genotype is 2/4 or 50%

The probability of an offspring have the dominant black coat is 4/4 or 100%

EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS

Bb and Bb The probability of an

offspring having a BB genotype is ¼ or 25%

The probability of an offspring having a Bb genotype is 2/4 or 50%

The probability of an offspring having a bb genotype is ¼ or 25%

EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS

¾ or 75% of the offspring resulting from this cross are predicted to have a black coat

¼ or 25% of the offspring are predicted to have a brown coat (recessive phenotype)

EXAMPLE 3: HETEROZYGOUS X HETEROZYGOUS

Genotypic ratio: the ratio of the genotypes that appear in offspring The probable genotypic ratio of the

monohybrid cross represented is 1BB: 2 Bb: 1 bb

Phenotypic ratio: the ratio of the offspring’s phenotypes The probable phenotypic ratio of the cross

is 3 black : 1 brown

EXAMPLE 4: TESTCROSS

In guinea pigs, both BB and Bb result in a black coat How would you determine whether a black

guinea pig is homozygous (BB) or heterozygous (Bb)?

Perform a testcross

Testcross: an individual of unknown genotype is crossed with a homozygous recessive individual Can determine the genotype of any

individual whose phenotype expresses the dominant trait

EXAMPLE 4: TESTCROSS

If the black guinea pig of unknown genotype is homozygous black, all offspring will be black

If the individual with the unknown genotype is heterozygous black, about half will be black

EXAMPLE 5: INCOMPLETE DOMINANCE

In Mendel’s pea-plant crosses, one allele was completely dominant over another Called complete dominance

In complete dominance, heterozygous plants and homozygous dominant plants are indistinguishable in phenotype Ex: PP and Pp produce purple flowered

plants

EXAMPLE 5: INCOMPLETE DOMINANCE

Sometimes, the F1 offspring will have a phenotype in between that of the parents Called incomplete dominance

Incomplete dominance occurs when the phenotype of a heterozygote is between the phenotypes determined by the dominant and recessive traits

EXAMPLE 5: INCOMPLETE DOMINANCE

Both the allele for red flowers (R) and the allele for white flowers (r) influence the phenotype Neither allele is

completely dominant When red flowers are

crossed with white flowers, all of the F1 offspring have pink flowers

EXAMPLE 5: INCOMPLETE DOMINANCE

100% of the offspring have the Rr genotype

The probable genotypic ratio is 1RR: 2 Rr: 1 rr Since neither allele is completely

dominant, the phenotypic ratio is 1 red: 2 pink: 1 white

EXAMPLE 6: CODOMINANCE

Codominance occurs when both alleles for a gene are expressed in a heterozygous offspring Neither allele is dominant or recessive, nor

do the alleles blend in the phenotype Example: blood types

Determined by two alleles

PREDICTING RESULTS OF DIHYBRID CROSSES

Dihybrid cross: a cross in which two characteristics are tracked The offspring are called dihybrids

HOMOZYGOUS X HOMOZYGOUS

Ex: Predict the results of a cross between a pea plant that is homozygous for round, yellow seeds and one that is homozygous for wrinkled, green seeds Round seeds (R) is dominant over wrinkled

seeds (r) Yellow seeds (Y) is dominant over green

seeds (y)

HOMOZYGOUS X HOMOZYGOUS

The Punnett Square used to predict the results of a cross between a parent of the genotype RRYY and a parent of the genotype rryy will contain 16 boxes Alleles are carried by

the male and female gametes

HOMOZYGOUS X HOMOZYGOUS

The genotype of all of the offspring of this cross will be heterozygous for both traits: RrYy All have round, yellow seed phenotypes

HETEROZYGOUS X HETEROZYGOUS

Cross two pea plants heterozygous for round, yellow seeds

Offspring are likely to have nine different genotypes

HETEROZYGOUS X HETEROZYGOUS

These 9 genotypes will result in pea plants that have the following four phenotypes: 9/16 that have round, yellow seeds

(genotypes RRYY, RRYy, RrYY, and RrYy) 3/16 that have round, green seeds (genotypes

Rryy and Rryy) 3/16 that have wrinkled, yellow seeds

(genotypes rrYY and rrYy) 1/16 that have wrinkled, green seeds

(genotype rryy)