Biology

Background: Deduced the fundamental principles of

genetics by breeding garden peas. Known as the “Father of Genetics” Was a monk that lived and worked in an

abbey in Austria. In a paper in 1866, Mendel correctly argued

that parents pass on to their offspring discrete heritable factors.

In his paper, he stressed that the heritable factors (today called genes) retain their individuality generation after generation.

Experiments: Chose to study garden peas because he was

familiar with them from his rural upbringing, they were easy to grow, and they came in many readily distinguishable varieties.

Also, he was able to exercise strict control over pea plant matings.▪ That is, sperm-carrying pollen grains released from

the stamens land on the egg-containing carpel of the same flower.

▪ He used a small bag to cover the flower to ensure self-fertilization.

Experiments (continued) Due to their anatomical nature (petals of pea

flower almost completely enclose the stamen and carpel), pea plants usually self-fertilize in nature.

He was also able to ensure cross-fertilization ▪ Fertilization of one plant by pollen from a different

plant. Through his methods, Mendel could always

be sure of the parentage of new plants

He chose seven characteristics, that occur in two distinct forms ,to study:▪ Flower color (purple, white)▪ Flower position (axial, terminal)▪ Seed color (yellow, green)▪ Seed shape (round, wrinkled)▪ Pod shape (inflated, constricted)▪ Pod color (green, yellow)▪ Stem length (tall, dwarf)

Mendel worked with his plants until he was sure he had true-breeding varieties. ▪ For instance, he identified a purple-flowered

variety that, when self-fertilized, produced offspring that all had purple flowers.

He then asked, What offspring would result if plants with purple flowers and plants with white flowers were cross-fertilized?▪ The offspring of two different varieties are called

hybrids, and the cross-fertilization itself is referred to as a hybridization, or simply a cross.

▪ The true-breeding parental plants are called the P generation ( P for parental).

▪ The offspring of the P generation are called the F1 generation (F for filial, the Latin word “son”)

▪ When the F1 self-fertilize or fertilize with each other, their offspring are called the F2 generation.

Mendel performed many experiments in which he tracked the inheritance of characteristics that occur in two forms, such as flower color.

Monohybrid cross: When you’re looking only at one trait

(ex, flower color)

Mendel performed a monohybrid cross between a pea plant with purple flowers and one with white flowers.▪ The F1 offspring all had purple flowers (not a lighter purple,

has predicted by a “blending” hypothesis.)▪ Was the white gene lost?

▪ By mating the F1 plants, Mendel found the answer to be NO!▪ Out of 929 F2 plants, Mendel found that 705 (about ¾) had

purple flowers and 224 (about ¼) had white flowers, a ratio of about three plants with purple flowers to one with white flowers in the F2 generation (3:1)

▪ The heritable factor for white flowers did not disappear in the F1 plants, but the purple-flower factor was the only one affecting the F1 flower color.

▪ The F1 plants must have carried two factors for the flower-color characteristic, one for purple and one for white.

Mendel observed these same patterns of inheritance for six other pea plant characteristics.

From these results, he developed four hypotheses, which we will describe using modern terminology (such as “gene” instead of “heritable factor”):

Hypothesis 1: There are alternative forms of genes

that account for variations in inherited characteristics.

For example, the gene for flower color in pea plants exists in two forms, one for purple and the other for white.

The alternative versions of a gene are now called alleles.

Hypothesis 2: For each characteristic, an organism inherits

two alleles, one from each parent. These alleles may be the same ore different.

An organism that has two identical alleles for a gene is said to be homozygous for that gene (and is called a homozygote).

An organism that has two different alleles for a gene is said to be heterozygous for that gene (and is called a heterozygote)

Hypothesis 3: If the two alleles of an inherited pair differ,

then one determines the organism’s appearance is called the dominant allele; the other has no noticeable effect on the organism’s appearance and is called the recessive allele.

We use upper-case letters to represent dominant alleles and lowercase letters to represent recessive alleles.

Hypothesis 4: A sperm or egg carries only one allele

for each inherited trait because allele pairs separate (segregate) from each other during the production of gametes.

This statement is now known as the law of segregation.

When sperm and egg unite at fertilization, each contributes its allele, restoring the paired condition in the offspring.

Also…Law of independent assortment –

during gamete formation (meiosis), alleles of DIFFERENT traits are arranged independently form one another

The right hand side of the diagram on the previous slide explains the results of Mendel’s experiment. In this example, P represents the dominant

allele (for purple flowers) and p represents the recessive allele (for white flowers).

At the top, you see the alleles carried by the parental plants both were true-breeding.

Mendel proposed that one parent had two alleles for purple flowers (PP) and the other had two alleles for white flowers (pp)

Mendel’s hypotheses also explain the 3:1 ratio in the F2 generation; because the F1 hybrids are Pp, they make gametes P and p in equal numbers.

You can see the possible gamete combinations using a Punnett square.▪ Used to make predictions about the possible

phenotypes and genotypes of offspring.

Consistent with hypothesis 4, the gametes of Mendel’s parental plants each carried one allele; thus, the parental gametes in Figure 9.3B are either P or p.

As a result of fertilization, the F1 hybrids each inherited one allele for purple flowers and one for white.

Hypothesis 3 explains why all of the F1 hybrids are (Pp) had purple flowers; the dominant P allele has its full effect in the heterozygote, while the recessive p allele had no effect on flower color.

Genotype vs. Phenotype Phenotype

▪ For example, purple or white flowers.▪ Term used to describe an organism’s appearance, or

expressed physical traits.▪ Phenotypic ratio- ratio of the phenotypes of the

offspring.▪ Example, the ratio of purple flowers to white flowers is 3:1

Genotype▪ For example, PP, Pp, or pp.▪ Term used to describe an organism’s genetic makeup.▪ Genotypic ratio- ratio of the genotypes of the offspring.

▪ Example, the 1:2:1 is the ratio of PP, Pp, pp

Remember, two homologous chromosomes may bear either the same alleles or different ones. Thus, we see the connection between Mendel’s laws and homologous chromosomes: Alleles (alternate forms) of a gene reside at the same locus on homologous chromosomes.

Test Cross: a mating between an individual of

unknown genotype and a homozygous recessive individual.

Mendel used testcrosses to determine whether he had true-breeding varieties of plants.

Continues to be an important tool of geneticists for determining genotypes.

Dihybrid cross: Results from a mating of parental varieties differing in two

characteristics. For example: Mendel crossed homozygous round yellow

seeds (RRYY) with plants having wrinkled green seeds (rryy).▪ All of the offspring in the F1 generation had round yellow

seeds; which raised the question: are the two characteristics transmitted from parent to offspring as a package, or was each characteristic inherited independently of the other?

▪ The question was answered when Mendel allowed fertilization to occur among the F1 plants the offspring supported the idea that the two seed characteristics segregated independently.▪ Offspring had nine different genotypes, and four different

phenotypes with 9:3:3:1 ratio.

Mendel’s results supported the hypothesis that each pair of alleles segregates independently of the other pairs of alleles during gamete formation Mendel’s Law of Independent Assortment

Although Mendel’s laws are valid for all sexually reproducing organisms, they stop short of explaining some patterns of genetic inheritance.

In fact, for most sexually reproducing organisms, cases where Mendel’s laws can strictly account for the pattern of inheritance are relatively rare.

More often, the inheritance patterns are more complex…

Complete dominance The dominant allele had the same

phenotype whether present in one or two copies.

Incomplete dominance The F1 hybrids have an appearance in

between the phenotypes of the two parental varieties.

For example, red snapdragons crossed with white snapdragons produced hybrid flowers in the F1 with pink flowers. This third phenotype results from flowers

of the heterozygote having less pigment than the red homozygotes.

The F2 generation would then have a 1:2:1 ratio of red, pink, white.

In humans, an example involves the condition hypercholestrolemia, dangerously high levels of cholesterol in the blood, caused by a recessive allele(h), due to a lack of LDL receptors.

hh individuals have about 5 times the normal amount of blood cholesterol and may have heart attacks as early as age 2.

Normal individuals are HH. Heterozygotes have blood cholesterol about twice

normal. Usually prone to atherosclerosis, the blockage of

arteries by cholesterol buildup in artery walls, and they may have heart attacks from blocked heart arteries by their mid-30s.

Both alleles are expressed in the heterozygous individual

Different from incomplete dominance, which is the expression of one intermediate trait

Can be seen in blood type

Both alleles are expressed in the heterozygous individual

Different from incomplete dominance, which is the expression of one intermediate trait

Can be seen in blood type

The ABO blood group phenotype in humans involves three alleles of a single gene.

These three alleles, in various combinations, produce four phenotypes: a person’s blood group may be either O, A, B, or AB.

These letters refer to two carbohydrates, designated A and B, that may be found on the surface of red blood cells.

A person’s red blood cells may have carbohydrate A (type A blood), carbohydrate B (type B), both (type AB), or neither (type O).

Matching compatible blood groups is critical for safe blood transfusions.

If a donor’s blood cells have carbohydrate (A or B) that is foreign to the recipient, then the recipient’s immune system produces blood proteins called antibodies that bind specifically to the foreign carbohydrates and cause donor blood cells to clump together, potentially killing the recipient.

Four blood groups result from various combinations of the three different alleles, symbolized as IA, IB, and i.

Each person inherits one of these alleles from each parent. IA and IB are dominant to the i allele, but are codominant

to each other = both alleles are expressed in the heterozygote IAIB , who have the blood type AB

There are six possible genotypes: IAIA and IAi= A IBIB and IBi= B IAIB= AB ii = O

Most genes influence multiple characteristics, a property called pleiotropy.

An example of pleiotropy in humans is sickle-cell disease Refer to p. 168

Polygenic inheritance is the additive effects of two or more genes on a single phenotypic characteristic Examples include human skin color and

height Different then pleiotropy, in which a

single gene affects several characteristics

The frequency of traits with polygenic inheritance follow the shape of a bell curve.

Besides bearing genes that determine sex, the sex chromosomes also contain genes for characteristics unrelated to femaleness and maleness.

Sex-linked genes are genes located on either sex chromosomes, although in humans the term has historically referred specifically to a gene on the X chromosome. ***Be careful not to confuse the term sex-linked gene with the term linked genes!!!***

Refer to figure 9.23A-D on p. 176

Many characteristics result from a combination of heredity and environment.

For example, in humans nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests.

It is becoming clear that human phenotypes—such as risk of heart disease and cancer and susceptibility to alcoholism and schizophrenia—are influenced by both genes and environment.

Simply spending time with identical twins will convince anyone that environment, and not just genes, affect a person’s traits.

However, only genetic influences are inherited…cannot pass on environmental influences to future generations!

Pedigree is a family tree used to study how particular human traits are inherited.

It is analyzed using logic and the Mendelian laws

1. Determine the mode of inheritance: dominant, or recessive, sex-linked or autosomal

2. Determine the probability of an affected offspring for a given cross.

Is it a dominant pedigree or a recessive pedigree? 1. If two affected people have an unaffected child, it must

be a dominant pedigree: D is the dominant allele and d is the recessive allele. Both parents are Dd and the normal child is dd.

2. If two unaffected people have an affected child, it is a recessive pedigree: R is the dominant allele and r is the recessive allele. Both parents are Rr and the affected child is rr.

3. If every affected person has an affected parent it is a dominant pedigree.

1 2 3 4 5 6 7 8 9 10

1 2

I

1 2 3 4 5 6

II

III

1. All unaffected are dd. 2. Affected children of an affected parent and an

unaffected parent must be heterozygous Dd, because they inherited a d allele from the unaffected parent.

3. The affected parents of an unaffected child must be heterozygotes Dd, since they both passed a d allele to their child. (also called carriers)

4. If both parents are heterozygous Dd x Dd, their affected offspring have a 2/3 chance of being Dd and a 1/3 chance of being DD.

1. all affected are rr. 2. If an affected person (rr) mates with an unaffected

person, any unaffected offspring must be Rr heterozygotes, because they got a r allele from their affected parent.

3. If two unaffected mate and have an affected child, both parents must be Rr heterozygotes.

4. Recessive outsider rule: outsiders are those whose parents are unknown. In a recessive autosomal pedigree, unaffected outsiders are assumed to be RR, homozygous normal.

5. Children of RR x Rr have a 1/2 chance of being RR and a 1/2 chance of being Rr. Note that any siblings who have an rr child must be Rr.

6. Unaffected children of Rr x Rr have a 2/3 chance of being Rr and a 1/3 chance of being RR.

In any pedigree there are people whose parents are unknown. These people are called “outsiders”, and we need to make some assumptions about their genotypes.

Sometimes the assumptions are proved wrong when the outsiders have children. Also, a given problem might specify the genotype of an outsider.

Outsider rule for dominant pedigrees: affected outsiders are assumed to be heterozygotes. (or carriers)

Outsider rule for recessive pedigrees: unaffected (normal) outsiders are assumed to be homozygotes.

Both of these rules are derived from the observation that mutant alleles are rare.

All unaffected individuals are homozygous for the normal recessive allele.

Example: Huntington’s Disease

Look for: Trait in every generation

Once leaves the pedigree does not return Every person with the trait must have a

parent with the trait Males and females equally affected

Look for: Skips in generation Unaffected parents can have affected

children Affected person must be homozygous Males and females affected equally

The recessive gene is located on 1 of the autosomes Letters used are lower case ie bb Unaffected parents (heterozygous) can produce affected

offspring (if they get both recessive genes ie homozygous) Inherited by both males and females Can skip generations If both parents have the trait then all offspring will also

have the trait. The parents are both homozygous. E.g. cystic fibrosis, sickle cell anaemia, albanism

Genes are carried on the sex chromosomes (X or Y)

Sex-linked notation XBXB normal female XBXb carrier female XbXb affected female XBY normal male XbY affected male

Mothers pass their X’s to both sons and daughters If the mother has an X- linked dominant trait and is

homozygous (XAXA) all children will be affected If Mother heterozygous (XAXa) 50% chance of each

child being affected Fathers pass their X to daughters only.

Affected males pass to all daughters and none of their sons ▪ Genotype= XAY

Normal outsider rule for dominant pedigrees for females, but for sex-linked traits remember that males are hemizygous and express whichever gene is on their X.

XD = dominant mutant allele Xd = recessive normal allele E.g. dwarfism, rickets, brown teeth enamel.

Look for:More males being affectedAffected males passing onto all

daughter (dominant) and none of his sons

Every affected person must have an affected parent

Look for: More males being affected Affected female will pass onto all

her sons Affected male will pass to

daughters who will be a carrier (unless mother also affected)

Unaffected father and carrier mother can produce affected sons

Examples: Hemophilia, Colorblindness