observing patterns in inherited traits · • alleles are different molecular forms of a gene with...

Observing Patterns in Inherited Traits

Chapter 11

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

11.1 Mendel, Pea Plants, and Inheritance Patterns

Recurring inheritance patterns are observable outcomes of sexual reproduction

Before the discovery of genes, it was thought that inherited traits resulted from a blend of parental characters

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

Garden Pea Plant: Self Fertilization and Cross-Fertilization

Terms Used in Modern Genetics

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

chromosome• But remember, gene is a stretch of DNA

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


A mutation is a permanent change in a gene• May cause a trait to change• Alleles are different molecular forms of a gene

with each on a homologous chromosome – they can be identical or non-identical in effect

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


An individual with nonidentical alleles of a gene is heterozygous for that gene

An individual with identical alleles of a gene is homozygous for that gene


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)


Gene expression• The process by which information in a gene is

converted to a structural or functional part of a cell or body

• Expressed genes determine traits• Genotype is what is in the DNA (genes)• Phenotype is the physical expression of the

genes (what you see as the trait)


Genotype• The particular alleles an individual carries

Phenotype• An individual’s observable traits


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

Thus, when gametes are prepared, the two units are separated and either can, by chance alone, end up in the gamete

Testcrosses

Testcross• A method of determining if an individual is

heterozygous or homozygous dominant• An individual with unknown genotype is crossed

with one that is homozygous recessive (AA x aa) or (Aa x aa)

• Phenotypes of result will indicate the genotype of the unknown parent

Monohybrid Experiments

Monohybrid experiments• Testcrosses that check for a dominance

relationship between two alleles at a single locus• May be crosses between true breeding

(homozygous) individuals (AA x aa), or between identical heterozygotes (Aa x Aa)

Mendel’s Monohybrid Experiments

Mendel used monohybrid experiments to find dominance relationships among pea plant traits• When he crossed plants that bred true for white

flowers with plants that bred true for purple flowers, all F1 plants had purple flowers

• When he crossed two F1 plants, ¾ of the F2plants had purple flowers, ¼ had white flowers

Segregation of Alleles at a Gene Locus

Mendel’s Monohybrid Experiments

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• Shows the genotypes for all possible offspring

Construction of a Punnett Square

Phenotype Ratios in a Monohybrid Experiment

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 rather than both being in same gamete

11.2 Key ConceptsInsights from Monohybrid Experiments

Some experiments yielded evidence of gene segregation: When one chromosome separates from its homologous partner during meiosis, the alleles on those chromosomes also separate 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• Really another way of stating that chromosomes

with their genes are separated and independently go into resulting gametes – but of course Mendel knew nothing of this and did not state it this way

Dihybrid Experiments

Dihybrid experiments• Tests for dominance relationships between

alleles at two loci • Individuals that breed true for two different traits

are crossed (AABB x aabb)• F1 phenotype result is heterozygote AaBb• F1 cross is then AaBb x AaBb and

F2 phenotype ratio is 9:3:3:1 (four phenotypes)9 A_B_; 3 A_bb ; 3 aaB_; 1 aabb

Independent Assortment at Meiosis

Mendel’s Dihybrid Experiments

Fig. 11-9c, p. 175

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

11.3 Key ConceptsInsights from Dihybrid Experiments

Some experiments yielded evidence of independent assortment: Genes are typically distributed into gametes independently of other genes

11.4 Beyond Simple Dominance

Mendel focused on traits based on clearly dominant and recessive alleles; however, the expression patterns of genes for some traits are not as straightforward

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• Example ABO blood types below

Multiple allele systems• Genes with three or more alleles in a population• Example: ABO blood types• A, B and AB are codominant; O is recessive• A=IAIA or IAIO; B=IBIB or IBIO; AB=IAIB; O=IOIO

Codominance in ABO Blood Types

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

Incomplete Dominance in Snapdragons

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

11.5 Linkage Groups

The farther apart two genes are on a chromosome, the more often crossing over occurs between them

Linkage group• All genes on one chromosome • Linked genes are very close together; crossing

over rarely occurs between them

Linkage and Crossing Over

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 temperaturesIt allows expression of melanin, a black color

Note how ice pack allowed black color development above

Effects of Altitude on Gene Expression

11.7 Complex Variations in Traits

Individuals of most species 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

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

observing patterns in inherited traits · • alleles are different molecular forms of a gene with...

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