Impacts, Issues:The Color of Skin

Like most human traits, skin color has a genetic basis; more than 100 gene products affect the synthesis and deposition of melanins

Mendel’s Experimental Approach

Mendel was a monk with training in plant breeding and mathematics

He studied the garden pea (Pisum sativum), which breeds true for a number of traits

Terms Used in Modern Genetics

Genes• Heritable units of info about traits• Parents transmit genes to offspring• Each gene has a specific locus on a

chromosome

Diploid cells (chromosome number 2n) have pairs of genes on homologous chromosomes

Terms Used in Modern Genetics

A mutation is a permanent change in a gene• Alleles are different molecular forms of a gene

A hybrid has nonidentical alleles for a trait• Offspring of a cross between two individuals that

breed true for different forms of a trait are hybrids

Terms Used in Modern Genetics

Nonidentical alleles of a gene is heterozygous

Identical alleles of a gene is homozygous

Terms Used in Modern Genetics

An allele is dominant if its effect masks the effect of a recessive allele paired with it• Capital letters (A) signify dominant alleles;

lowercase letters (a) signify recessive alleles• Homozygous dominant (AA)• Homozygous recessive (aa)• Heterozygous (Aa)

Terms Used in Modern Genetics

Gene expression• Process by which info in a gene is converted to a

structural or functional part • Expressed genes determine traits

Terms Used in Modern Genetics

Genotype• The particular alleles an individual carries

Phenotype• An individual’s observable traits

Terms Used in Modern Genetics

P stands for parents, F for filial (offspring)

F1: First generation offspring of parents

F2: Second generation offspring of parents

11.1 Key ConceptsWhere Modern Genetics Started

Gregor Mendel gathered the first experimental evidence of the genetic basis of inheritance

His meticulous work gave him clues that heritable traits are specified in units

The units, which are distributed into gametes in predictable patterns, were later identified as genes

11.2 Mendel’s Law of Segregation

Garden pea plants inherit two “units” of information for a trait, one from each parent

Fig. 11-5, p. 172

homozygous dominant parent

homozygous recessive parent

(chromosomes duplicated before

meiosis)

meiosis I

meiosis II

(gametes) (gametes)

fertilization produces heterozygous offspring

Calculating Probabilities

Probability• A measure of the chance that a particular

outcome will occur

Punnett square• A grid used to calculate the probability of

genotypes and phenotypes in offspring

Construction of a Punnett Square

Phenotype Ratios in a Monohybrid Experiment

Phenotype Ratios in a Monohybrid Experiment

Fig. 11-7b, p. 173

aaF1 offspring

True-breeding homozygous recessive parent plant

a aAa Aa

A Aa AaAA

A Aa Aa

True-breeding homozygous dominant parent plant

Aa Aa

B A cross between two plants that breed true for different forms of a trait produces F1 offspring that are identically heterozygous.

Mendel’s Law of Segregation

Mendel observed a phenotype ratio of 3:1 in the F2 offspring of his monohybrid crosses• Consistent with the probability of the aa genotype

in the offspring of a heterozygous cross (Aa x Aa)

This is the basis of Mendel’s law of segregation• Diploid cells have pairs of genes on pairs of

homologous chromosomes • The two genes of each pair separate during

meiosis, and end up in different gametes

11.3 Mendel’s Law of Independent Assortment

Mendel’s law of independent assortment• Many genes are sorted into gametes

independently of other genes

Fig. 11-8, p. 174

One of two possible alignments The only other possible alignment

a Chromosome alignments at metaphase I:

A A a a A A a a

B B b b b b B B

b The resulting alignments at metaphase II:

A A a a A A a a

B B b b b b B B

c Possible combinations of alleles in gametes:

B A A B b a a b b A A b B a a B

AB ab Ab aB

• Dihybrid Cross:• In humans, there is a gene that controls

formation (or lack thereof) of muscles in the tongue that allow people with those muscles to roll their tongues, while people who lack those muscles cannot roll their tongues. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste certain substances is also genetically controlled. Let’s let R represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t represent non-tasting.

Suppose 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 children Draw the Punnett square that predicts what their children will be. If the man is both Rr and Tt (How do we know that?), he would be RrTt and so could produce gametes with either R or r and either T or t (one allele for each gene). There are two choices for the first trait (R or r). No matter which of those go into a given sperm, there are still two choices for the second trait (T or t).

Woman RRtt

Man RrTt

RT Rt rT rt

Rt RRTt RRtt RrTt Rrtt

Rt

Rt

Rt

Mendel’s Law of Independent Assortment

Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits

Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis

Codominance in ABO Blood Types

Codominance• Two nonidentical alleles of a gene are both fully

expressed in heterozygotes, so neither is dominant or recessive

• May occur in multiple allele systems

Multiple allele systems• Genes with three or more alleles in a population• Example: ABO blood types

Fig. 11-10, p. 176

AA BB

Genotypes: AO AB BO OO

Phenotypes (Blood type): A AB B O

or or

Incomplete Dominance

Incomplete dominance• One allele is not fully dominant over its partner• The heterozygote’s phenotype is somewhere

between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring

Example: Snapdragon color• RR is red• Rr is pink• rr is white

Fig. 11-11a, p. 176

homozygous parent (RR) x

homozygous parent (rr)

heterozygous F1 offspring (Rr)

A Cross a red-flowered with a white-flowered plant, and all of the F1 offspring will be pink.

Fig. 11-11b, p. 176

R r

B Cross two F1 plants, and the three phenotypes of the F2 offspring will occur in a 1:2 :1 ratio:

R

RR Rr

r

Rr rr

Epistasis

Epistasis• Two or more gene products influence a trait• Typically, one gene product suppresses the effect

of another

Example: Coat color in dogs• Alleles B and b designate colors (black or brown)• Two recessive alleles ee suppress color

Epistasis in Coat Colors

Pleiotropy

Pleiotropy• One gene product

influences two or more traits

• Example: Some tall, thin athletes have Marfan syndrome, a potentially fatal genetic disorder

The Distance Between Genes

The probability that a crossover event will separate alleles of two genes is proportional to the distance between those genes

11.6 Genes and the Environment

Expression of some genes is affected by environmental factors such as temperature, altitude, or chemical exposure

The result may be variation in traits

Effects of Temperature on Gene Expression

Enzyme tyrosinase, works at low temperatures

11.7 Complex Variations in Traits

Individuals vary in some of their shared traits

Many traits (such as eye color) show a continuous range of variation

Continuous Variation

Continuous variation• Traits with a range of small differences• The more factors that influence a trait, the more

continuous the distribution of phenotype

Bell curve• When continuous phenotypes are divided into

measurable categories and plotted as a bar chart, they form a bell-shaped curve

Continuous Variation and the Bell Curve

Regarding the Unexpected Phenotype

Phenotype results from complex interactions among gene products and the environment• Enzymes and other gene products control steps

of most metabolic pathways• Mutations, interactions among genes, and

environmental conditions may affect one or more steps

11.4-11.7 Key ConceptsVariations on Mendel’s Theme

Not all traits appear in Mendelian inheritance patterns• An allele may be partly dominant over a

nonidentical partner, or codominant with it• Multiple genes may influence a trait; some genes

influence many traits• The environments also influences gene

expression