People have understood for centuries that certain traits are inherited (transmitted fromgeneration to generation) but did not understandhow it happened.

Mendel’s experiments answered many of thesequestions, because his experiments were socarefully planned, executed, and quantified.

Mendel worked with peasA. produced many offspringB. worked with plants that “bred true”, e.g.,plants with purple flowers always produced

plants with purple flowersC. He could control the breeding process

Also, he could self-fertilize the plants(“self-cross”)

D. He counted the offspring and analyzed the results

E. He bred plants so that they differed in only one characteristic, or only two…

F. For each trait there were only two possibleoutcomes

Example: Monohybrid cross (plants are identicalexcept for one characteristic, or trait)

Mendel examined seed shape. Plants had eithersmooth or wrinkled seeds. He crossedplants having smooth seeds with plantshaving wrinkled seeds.

Result: all of the plants had smooth seeds.

Next, he planted the seeds from this crossand self-fertilized them. What happened?

He collected over 7000 seeds and counted them.

5474 were smooth1850 were wrinkled

¾ were smooth, and ¼ were wrinkled

In Mendel’s terminology:

P1 (parents) Smooth X wrinkledF1 (first generation) all smoothF2 (second generation) 5474 smooth

1850 wrinkled

Mendel examined seven traits and always got thesame results. Interpretation:

The F1 always showed only one of the two parentaltraits, and always the same trait.

It didn’t matter which plant donated the pollen

The trait that “disappeared” in the F1 generationreappeared in about 25% of the F2’s

So traits did not blend, but remained unchangedfrom one generation to another

Mendel’s conclusions:

Traits (what we now call genes) are not alwaysexpressed.

Genes that are always expressed are calleddominant genes

Genes that are not expressed if a dominant geneis present are called recessive genes

The P1 plants and F1 plants have the sameappearance (smooth seeds) but havedifferent genes:

The P1 plants produce only smooth seeds,but the F1 plants produce smoothand wrinkled seeds

These plants have the same PHENOTYPE(outward appearance)

But have different GENOTYPES (geneticmakeup)

Each parent contributes the same amountof genetic information

Symbols for dominant and recessive traits

Upper case letter-dominant version of the gene

Lower case letter- recessive version

Smooth seeds are dominant, so S stands for smoothseeds

Wrinkled seeds are recessive, so s stands forwrinkled seeds

A plant that always produces smooth seedshas two S alleles. The smooth seed allele is dominant to the

wrinkled seed alleleSS=homozygous dominant

homozygous: both alleles are the samedominant: both alleles are dominant

A plant that always produces wrinkled seedshas two s alleles: homozygous recessive.

The F1 plants are Ss: heterozygous(one allele from each parent)

Mendel’s Law of Segregation

Each parent has two genes for a trait(allele: variant version of a gene)

Each gamete receives one of the two genes

Parent SS ss

Gametes S S s s

SsOffspring

Crosses involving two traits: Principle ofindependent assortment

Mendel worked with seeds on this

Yellow (Y) is dominant to green (y)Smooth (S) is dominant to wrinkled (s)

P1 cross: Smooth yellow (SSYY) withwrinkled green, (ssyy)

F1s are all smooth and yellow (SsYy)

He crossed these with each other:

SY Sy sY sySY

Sy

sY

sy

SSYY SSYy SsYY SsYy

SSYy SSyy SsYy Ssyy

SsYY SsYy ssYY ssYy

SsYy Ssyy ssYy ssyy

X = allSSYY ssyy SsYy

Out of 16 possible combinations;

9 have at least one copy of BOTH dominant alleles(S and Y)

3 have at least one copy of S and are homozygousrecessive for yy

3 have at least one copy of Y and are ss1 has recessive for ss and yy

So the s and y alleles were distributed randomlyinto gametes during meiosis(independent assortment)

We now know (and Mendel did not) that thishappens because genes are on chromo-somes.

Genes ChromosomesOccur in pairs (alleles)occur in pairs

(homologues)

Members of a gene pair Homologues separateseparate during meiosis during meiosis

Members of one gene pair Members of one pair assort independently of of chromosomesother gene pairs assort independentlyduring meiosis of others during meiosis

These laws apply to many examples of geneticinheritance. Variations have also beenobserved.

Multiple alleles

• More than two alleles in the population– (although any organism has two)– Both alleles may be expressed: codominant– Example : ABO blood groups

• Three alleles, IA, IB and I• Type O is recessive (ii)• Type A person could be IAIA or Iai• What is genotype for type B? type AB? Type O?

Blood types are inherited in Mendelian fashion.

You can use the format AO for a type A heterozygote,AA for a type A homozygote, etc.

If two type A parents have a type O child, what musttheir genotypes be?

AO and AO

Can a type O man father a type B child? If so, what is the genotype of the child?

Yes; type BOCan a type AB man and a type B woman have a type A child?

Yes, if the woman’s genotype is BO

Other effects on phenotype

Multiple alleles (continuous variation)

Pleiotropic effectscystic fibrosis, sickle cell anemia

Environmental effectstemperature sensitivity“risk factors”

Incomplete dominance

Penetrance; expressivity

Sex linkage-Morgan’s experiment

White-eyed male fly was crossed with a red-eyedfemale flyAll of the F1 (offspring had red eyes). F1 flies werecrossed with each other.

A 3:1 red:white ratio was observed- but all of the white-eyed flies were male.

The F1 females were test-crossed with the white-eyedmales

What is a testcross?

What is a testcross?Is an organism with the dominant phenotypehomozygous dominant or heterozygous?How can you find out? What kind of mating experimentwill tell you?Cross the organism with a homozygous recessiveorganism: A- X aa

What will be the result if the test organism is AA?All of the offspring will have the dominant phenotype.

What will be the result if the test organism is Aa?

Half of the offspring will have the dominant phenotypeAnd half will have the recessive phenotype.

Back to the testcross of F1 females with white-eyed males

What happened?

Phenotypic ratio was 1:1:1:1red-eyed femaleswhite-eyed femalesred-eyed maleswhite-eyed males

As expected

Why did recessive trait “disappear” in F1 females?

The eye-color trait is on the X chromosome.

Except for the sex chromosomes, the other(autosomal) chromosomes are homologous pairs. GeneticInformation on those chromosomes is inherited as pairs of alleles (homozygous or heterozygous).

Sex chromosomes: in flies and humans, females havetwo X chromosomes and males have one X and one Y.

when two gametes fuse, if both contain Xchromosomes, the offspring is female. If one gametecontains X and the other Y, the offspring is male.(independent assortment of chromosomes)

Implications:

X Y

X

X

XX XY

XX XY

So if a recessive allele is on the X chromosome, afemale needs two copies to have the recessive phenotype, but a male needs only one.

A female offspring must inherit which chromosomefrom her father? A male offspring must inherit which chromosome from his father?

The P fly cross: let W=red eyes and w=white eyes

(male) white-eyed(female) red-eyed

Xw Y

XW

XW

XWXwXWY

XWXw XWY

All of the F1 flies have red eyes.The females are heterozygous (“carriers”).The males have inherited the red-eye gene from theirmothers.

The F1 cross:Males: XWY

Females: XWXw

XW Y

XW

Xw

XWXW XWY

XWXw XwY

All of the females have red eyes.Half of the males have red eyes and half have white eyes.

Testcross of the F1 females (XWXw)

XWXw X XwY

Xw Y

XW

Xw

XWXw XWY

XwXw XwY

Red-eyed female:white-eyed female:red-eyed male:White-eyed male

1:1:1:1

In flies, white eye color is sex-linked recessive

Recessive, because red eyes are dominantto white

Sex-linked, because the gene is on the X chromosome

How can you tell if a characteristic is inherited in a X (sex)-linked recessive manner?

Males with the affected X chromosome, andHomozygous females, are affected.Phenotype is seen much more often in males.Affected males inherit the allele from their mothersand pass it on to their daughters.Daughters of affected males are usually heterozygousand thus unaffected.Sons of heterozygous mothers have a 50% chanceof inheriting the gene.

Some X-linked recessive traits in humans

• Color blindness (red or green)• Hemophilia• Duchenne muscular dystrophy• SCID (severe combined immune deficiency

syndrome

X-linked dominant traits

• Affected males produce all affected daughters and no affected sons

• A heterozygous affected female will transmit the gene to half of her children (male or female)

• About twice as many females as males are affected

• Few of these traits are known in humans

Genes on Y chromosome are passed only fromfather to son

Mitochondrial DNA is passed from mother to all offspring. Only daughters can pass on the same DNAto their offspring.

In humans, patterns of inheritance are studiedwith pedigree analysis

Pedigree analysis

Used in human genetic analysishumans don’t produce enough offspringfor counting analysis

Pedigree chart: a diagram that shows the membershipand ancestral relationships in a family

Pedigree analysis: use of family history to determinehow a trait is inherited; used in the study of humanheredity

Constructing a pedigree

= male= female= mating

= parents and children. Parents are the uppergroup, children the lower. From left to right,children are shown in birth order (so the sonis the youngest child).

or= unaffected by the trait in question

or = affected by the trait in question

A pedigree of three generations

III: most recentgeneration

II: parents

I. grandparents

youYour motherYour father

Inheritance of an autosomal dominant trait

Each affected child has at least one affected parent. Two affected parents can have an unaffected child.

Inheritance of an autosomal recessive trait

Two unaffected parents can have an affected child.

Inheritance of sex-linked recessive traits

Trait is seen much more often in males.Unaffected females may be “carriers” who pass theaffected X chromosome to their sons.Affected males pass the affected chromosome todaughters but not sons.

Inheritance of X-linked dominant traits

An affected father passes the trait to all of his daughtersand none of his sons.An affected woman has a 50% chance of passing the trait to both sons and daughters.