Chapter 8

8.1 Genes Are Particulate and Are Inherited According to Mendel’s Laws

8.2 Alleles and Genes Interact to Produce Phenotypes

8.3 Genes Are Carried on Chromosomes

8.4 Prokaryotes Can Exchange Genetic Material

How is hemophilia inherited, and why is it most frequent in males?

8.1

Genetics, the field of biology concerned with inheritance, has a long history as people were breeding horses and trees for desirable characteristics

Breeding of plants and animals had yielded two hypotheses by the mid-19th century:

Blending inheritance—gametes contained determinants (genes) that blended when gametes fused during fertilization (ink of different colors fused)

Particulate inheritance—each determinant was physically distinct and remained intact during fertilization

In the 1860s, the monk GregorMendel used the scientific method in studies of garden peas that clearly supported the particulate hypothesis.

Garden peas were easy to grow and manipulate: their flowers have both male and female sex organs (pistils and stamens) that produce gametes.

Peas normally self-fertilize, but male organs can be removed to allow fertilization with pollen from other flowers.

Characters—observable physical features (e.g., flower color, seed shape)

Traits—forms of a character (e.g., purple flowers, wrinkled seeds)

Mendel worked with true-breeding varieties—when plants of the same variety are crossed, all offspring have characters with the same traits.

Parental generation = P Resulting offspring = first filial

generation, or F1

If F1 plants self-pollinate, they produce the second filial generation, or F2.

“Hybrid” refers to offspring of crosses between organisms differing in one or more characters.

In the 1st experiment, Mendel crossed plants differing in just one character, producing monohybrids in the F1 generation.

The monohybrids were then allowed to self-pollinate to form the F2generation—a monohybrid cross.

Figure 8.1 Mendel’s Monohybrid Experiments (Part 2)

All seven crosses between varieties with contrasting traits gave the same kind of data.

Mendel called the trait that appeared in the F1 and was more abundant in the F2 the dominant trait, and the other trait recessive.

The ratio of dominant to recessive in the F2 was about 3:1.

Mendel proposed that the genetic determinants (genes) occur in pairs and are segregated in the gametes. Each plant has two genes for each character, one from each parent.

We now use the term diploid to describe having two copies of a gene, and haploid as having one copy.

Mendel concluded that each gamete contains one copy of each gene (haploid), the resulting zygote contains two copies (diploid), because it is produced by the fusion of two gametes.

Different traits arise because there can be different forms of a gene—now called alleles—for a particular character.

Phenotype—the physical appearance of an organism (e.g., round seeds)

Genotype—the genetic makeup (e.g., Rr)

Capital letters designate the dominant allele, lower case designate the recessive.

Round seeds can be the result of two different genotypes—RR or Rr.

True-breeding individuals have two copies of the same allele—they are homozygous for the allele (e.g., rr).

Heterozygous individuals have two different alleles (e.g., Rr).

Mendel’s first law:

The law of segregation states that the two copies of a gene separate when an individual makes gametes. Each gamete receives only one copy.

Gametes from an RR individual will all be R, gametes from an rr individual will all be r, and their offspring (F1) will all be Rr.

In the F2 generation, gametes will be R or r. Genotypes in the F2 generation can be predicted using a Punnett square,

which considers all possible gamete combinations.

The Punnett square predicts a 3:1 ratio of phenotypes in the F2 generation.

Mendel’s experimental values were remarkably close to this for all seven of the character traits he compared.

Genes are now known to be relatively short sequences of DNA found on the much longer DNA molecules that make up chromosomes.

Today, we can picture the different alleles of a gene segregating as chromosomes separate during meiosis I.

Genes determine phenotypes mostly by producing proteins with particular functions.

In many cases a dominant gene is expressed, and the recessive gene is mutated so that it is no longer expressed or produces a non-functional protein.

The wrinkled pea phenotype is due to a lack of starch branching enzyme 1 (SBE1), essential for starch synthesis.

Mendel verified his hypotheses by doing test crosses:

Cross F1 with known homozygote which has the recessive genotype (e.g., rr).

If the F1 individual was homozygous, all offspring will show the dominant trait. If the F1 was heterozygous, half of the offspring will have the recessive trait.

• Tested round peas of undetermined genotype by crossing them with wrinkled peas with a known genotype (homozygous recessive)

• If the plant being tested is homozygous RR

• If the plant being tested in heterozygous Rr

All dominant…or half the sees from the cross will be wrinkled

Mendel’s next experiments tested whether genes are distributed independently in the offspring. He crossed peas that differed in two

characters—seed shape and color True-breeding parents: RRYY—round yellow seeds rryy—wrinkled green seeds

Dihybrid Cross - Bozeman

F1 generation is RrYy—all round yellow.

Crossing the F1 generation (all identical double heterozygotes) is a dihybrid cross.

Mendel asked whether, in the gametes produced by RrYy, the traits would be linked or segregate independently.

Two possible outcomes of the dihybrid cross: If the alleles were linked, gametes

would be RY or ry; F2 would have three times more round yellow than wrinkled green. The ratio would be 3:1

If independent, gametes could be RY, ry, Ry, or rY. F2 generation would have nine

different genotypes, and the four phenotypes would be in a 9:3:3:1 ratio.

Figure 8.5 Independent Assortment

Mendel’s dihybrid crosses supported the second prediction.

Mendel’s second law is the law of independent assortment: Alleles of different genes assort independently during gamete formation.

This law does not always apply to genes near each other on the same chromosome, but chromosomes do segregate independently.

Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 1)

Figure 8.6 Meiosis Accounts for Independent Assortment of Alleles (Part 2)

One reason Mendel was successful was his use of large sample sizes.

By counting many progeny from each cross, he was able to see clear patterns.

After his work became recognized, geneticists began using probability calculations to predict expected ratios of genotypes and phenotypes and statistics to determine whether actual results matched the predictions.

Probability

If an event is certain to happen, its probability = 1

If an event cannot possibly happen, its probability = 0

All other events have a probability between 0 and 1

Two coin tosses are independent events; each will come up heads ½ of the time.

The probability that both coins will come up heads is:

½ × ½ = ¼

Multiplication rule: the probability of two independent outcomes occurring together is found by multiplying the individual probabilities.

In a monohybrid cross:

After self-pollination of an F1 Rr, the probability that the F2 offspring will have the genotype RR is

½ × ½ = ¼

Probability that offspring will have the rr genotype is the same.

Probability that offspring will be heterozygous:

There are two ways to get a heterozygote— Rr or rR.

Addition rule: probability of an event that can occur in two or more different ways is the sum of the individual probabilities of those ways:

¼ + ¼ = ½

Human pedigrees can show Mendel’s laws.

Humans have few offspring; pedigrees do not show the clear proportions that the pea plants showed.

But pedigrees can show patterns and can be used to determine whether a rare allele is dominant or recessive.

Pattern of inheritance if a rare allele is dominant:

Every person with the abnormal phenotype has an affected parent.

Either all (if homozygous parent) or half (if heterozygous parent) of offspring in an affected family are affected.

Pattern of inheritance if the rare allele is recessive:

Affected people often have two unaffected parents.

In an affected family, one-fourth of children of unaffected parents are affected. The parents are heterozygous carriers.