chapter 14

CHAPTER 14

INTRODUCTION TO GENETICS

MODELS OF HEREDITY

1. BLENDING MODEL- genetic material contributed by the two parents

mixes - over many generations, a freely mating population

will give rise to a uniform population of individuals- everyday observation contradicts this model- does not explain why traits sometimes skip

generations

2. Particulate model (the gene idea)- parents pass on discrete heritable units (genes)

that retain their separate identities in offspring

Modern genetics began with the work of Gregor Mendel, who documented this particulate model of inheritance

Figure 14.0x Mendel

Figure 14.0 Painting of Mendel

GREGOR MENDEL

Mendel begin breeding garden peas around 1857 to study inheritance

- there was a long tradition of breeding plants at the monastery where he lived

- he probably chose to work with peas because there are many varieties

CHARACTER- a heritable feature that varies among individuals, such as flower color

TRAIT- a variant for a character, such as purple or white flowers

By using peas, Mendel was also able to control which plants mated

- each pea flower has both male (stamens) and female (carpel) parts

- these plants usually self-fertilize

Mendel cross-pollinated the plants:- he removed immature stamens from a plant

before they produced pollen- he then dusted the carpel of with pollen from

another flower

Figure 14.1 A genetic cross

Mendel began all of his experiments with TRUE-BREEDING

- when the plants self-pollinate, all offspring are of the same variety

Mendel crossed 2 true-breeding varieties (example: white vs. purple flowers)

- the parents are called the P1 generation

- their hybrid offspring are called the F1 generation

- allowing the F1 generation to self-pollinate produced the F2 generation

RESULTS OF THE EXPERIMENTS

When Mendel crossed pure purple and pure white flowered plants, he got all purple flowers in the F1 generation

- if the blending model had been correct, these flowers should have been pale purple

What happened to the white?- when Mendel allowed F1 plants to self-pollinate,

white flowers reappeared

Figure 14.2 Mendel tracked heritable characters for three generations

- his results worked out to about 3 purple to 1 white flower

Mendel reasoned that the factor for white flowers did not disappear in the F1 plants, but only the purple-flower factor was affecting flower color in this generation

- Mendel called the purple color a DOMINANT TRAIT and the white color a RECESSIVE TRAIT

Mendel observed other characters with the same results:

Flower position- axial or terminalSeed color- yellow or greenSeed shape- round or wrinkledPod shape- inflated or constrictedPod color- green or yellowStem length- tall or dwarf

Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants

LAW OF SEGREGATION

Mendel developed a hypothesis to explain his results that can be broken down into 4 parts:

1. Alternative versions of genes account for variations in inherited characters

- ALLELES- alternative versions of a geneEx: Gene is flower color, alleles are purple

and white

- the purple allele and the white allele are 2 DNA variations possible at the flower color locus on one of a pea plant’s chromosomes

Figure 14.3 Alleles, alternative versions of a gene

2. For each character, an organism inherits two alleles, one from each parent

- RECALL: a diploid organism has homologous pairs of chromosomes, one from each parent

3. If the 2 alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance

4. The two alleles for each character segregate (separate) during gamete production

- an ovum and a sperm each get only one of the two alleles that are present in the somatic cells of the organism

- this is where the name of the law, the LAW OF SEGREGATION, comes from

PUNNETT SQUARE- a diagram used to predict the results of a genetic cross

Figure 14.4 Mendel’s law of segregation (Layer 2)

SOME IMPORTANT VOCABULARY

HOMOZYGOUS- an organism having a pair of identical alleles for a character

Ex: a pea plant that is true-breeding for purple flowers (PP)

- can also be for white flowers (pp)HETEROZYGOUS- an organism having 2 different

alleles for a gene- for the flowers, would be Pp- produces a purple flower

PHENOTYPE- an organism’s traits- this is the PHYSICAL APPEARANCE or

ABILITY- for the flowers, the phenotypes are either purple

or whiteGENOTYPE- an organism’s genetic makeup- for the flowers, the phenotypes could be PP, pp,

or Pp

Figure 14.5 Genotype versus phenotype

TESTCROSS

A TESTCROSS is a genetic cross performed when the genotype of one of the parents is unknown

- the genotype of the parent can be determined by looking at the offspring

LAW OF INDEPENDENT ASSORTMENT

The Law of Segregation was derived by performing breeding experiments using only a single character

- the F1 hybrids produced in these crosses are called MONOHYBRIDS (Monohybrid crosses)

Mendel also performed DIHYBRID CROSSES- crosses involving 2 separate characters

Mendel studied seed color and seed shape- he knew that the allele for yellow seeds is

dominant over green seeds, and the allele for round seeds is dominant over wrinkled seeds

- he crossed 2 true-breeding plants that differed in BOTH characters:

YYRR x yyrr

Mendel wondered if the 2 characters, seed color and seed shape, were transmitted from parents to offspring as a package

- in other words, will Y and R alleles always stay together?

Or- are they inherited independently of each

other?

For both possibilities, the F1 plants were heterozygous for both traits:

YyRr- Mendel needed to see what would happen

when these plants self-pollinated (in the F2 generation)

THE RESULTS

Experimental results supported the hypothesis that each character is independently inherited

- the 2 alleles for seed color segregate independently of the 2 alleles for seed shape

- Mendel always ended up with the 9:3:3:1 phenotypic ratio

The independent segregation of each pair of alleles during gamete formation is now called the LAW OF INDEPENDENT ASSORTMENT

PROBABILITY

RECALL:The probability scale ranges from 0 to 1- an event certain to occur has a probability of 1- an event certain NOT to occur has a probability of

0- with a coin, the chance of tossing heads is ½, tails

is ½

This can be applied in fertilization:- the ovum has ½ chance of carrying a

dominant allele, and ½ chance of carrying a recessive allele

- the same odds apply to the sperm- like 2 separate coin tosses, allele segregation

occurs 2 independent events

Rule of Multiplication

“What is the chance that 2 coins tossed simultaneously will land heads up?”

- we will find the probability for each independent event and then multiply these events together

½ x ½ = ¼

“An F1 plant is Pp for purple flower color. What is the probability that an F2 plant will have white flowers?”

- both ovum and sperm must carry a p½ x ½ = ¼

“What is probability of an F2 plant having genotype YYRR?”

- probability of YR is ¼¼ x ¼ = 1/16

Rule of Addition

“What is the probability that an F2 plant from a monohybrid cross will be heterozygous?”

- there are 2 ways that this can occur:* the dominant allele can come from the ovum and the recessive from the sperm, or vice versa

- to find the probability that an event can occur in 2 or more different ways, we add the probabilities

¼ + ¼ = ½

We can combine these rules for more complicated crosses:

PpYyRr x PpyyrrCalculate the fraction of offspring predicted to

show the recessive phenotypes for at least 2 of the 3 traits :

Ppyyrr, ppYyrr, ppyyRr, PPyyrr, ppyyrr- we will combine the rules

PpYyRr x Ppyyrr

ppyyRr: ¼ (prob. of pp) x ½ x ¼ = 1/16ppYyrr: ¼ x ½ x ½ = 1/16Ppyyrr: ½ x ½ x ½ = 2/16PPyyrr: ¼ x ½ x ½ = 1/16ppyyrr: ¼ x ½ x ½ = 1/16___________________________________Chance of at least 2 = 6/16 recessive traits or 3/8

Try it:PPYyrr x PpyyRr

“What is the probability of getting dominant phenotypes for at least 2 of the 3 traits?”