Introduction to Genetics

Chapter 11 (M)

Genetics and InheritanceGenetics the study of

heredity that deals with the transmission of traits or characteristics from one generation to another

Inheritance The reception of traits by transmission from parent to offspring

Prehistoric Times

Little is known when humans first recognized the importance of genetics

The breeding of horses, cattle, and various breeds of dogs began around 8000 B.C. and 1000 B.C.

Plants such as corn, wheat, and rice was cultivated in Mexico around 5000 B.C.

The History Of Genetics

Pythagoras 500 B.C., a Greek philosopher, stated that human life began with male and female fluids

The History Of Genetics

Aristotle furthered this idea and suggested that these fluids, or “semens,” were actually purified blood—therefore, blood must be part of heredity.

•The theory of Homunculus -17th century

•sex cells contained a complete miniature adult, perfect in form

•This statement was popular way into the 18th century

Small individual

One Bizarre Theory

Blending Hypothesis 1800’s This stated that both the genetic material

from the mother and father were blended in order to produce an offspring

Parents Red Flower X Yellow Flower Offspring Orange (offspring all orange)

Exceptions Red yellow ,etc Theory discarded, If blending occurred all

extreme characteristics would disappear from the population

•Considered to be the “father of genetics•Austrian monk, teacher and mathematician•Expt. approach to genetics•Particulate Hypothesis

Gregor Mendel-1800’s

Particulate Hypothesis

Parents pass on to their offspring separate and distinct factors (genes) that are responsible for inherited traits

Used pea plants to study this

Why peas? Rapid reproduction

rate Presence of

distinctive traits Closed structure of

flowers (each pea plant has male (stamens) and female (carpal)

sexual organs) allows self-fertilization

Cross-fertilization

One plant is fertilized by another

Terminology

P Generation parents

F1 Generation first filial

F2 Generation second filial

P X P

F1 F1 F1

F2

Alternative forms of a gene which determines a trait.

Allele

Alleles cont.Uppercase (Capital) letters for

dominant traitsLowercase letters for

recessive traitsEx: Tall = T short = t,

expressed in pairs TT, Tt, tt

Phenotype Physical appearance

Genotype Genetic makeupDominant trait that is easily

observedRecessive trait that is often

maskedHomozygous2 alleles for a

trait are identical TT or ttHeterozygous – 2 alleles for a

trait are not identical Tt

Inheritance Follows Rules of Chance

Mendel began experiments to track the inheritance of characters in pea plants

Results led him to formulate several hypotheses

Seven Traits Studied by Mendel

Terminology

P Generation parents

F1 Generation first filial

F2 Generation second filial

P X P

F1 F1 F1

F2

Mendel’s Experiment Crossed pure

purple and a pure white flower (P generation) =F1

generation All F1 plants

(purple) are crossed by self pollination = F2 generation yields ¾ purple and ¼ yellow

Mendel’s Hypotheses

1. There are alternate forms of genes

2. For each character an organism has two alleles

3. Alleles are either dominant or recessive

4. Alleles segregate during formation of gametes

Law of Dominance

When organisms pure for contrasting traits are crossed,all their offspring will show thedominant trait

Probability Fractions or ratios that will predict that

an event will occur

Punnett Square

Diagram which shows the possible outcome of a cross

Monohybrid Cross

Using a single trait – crossing a pure bred Tall (TT) with a pure bred short (tt) plant

Mating 2 heterozygous black (Bb) rabbits

Test Cross• Individual with dominant

phenotype not possible to predict the genotype run a test cross with individual with recessive phenotype to determine the allele

Dominant Phenotype purple flower (genotype PP or Pp)

Recessive Phenotype white flower ( genotype pp)

Test Cross

Law of IndependentAssortment

Each pair of alleles segregates into gametes independently

Independent Assortment Mendel followed the inheritance of two

different characters ( dihybrid cross)The allele for yellow seeds (Y) is dominant

to the allele for green seeds (y).The allele for round seeds (R) is dominant

to the allele for wrinkled seeds (r). He crossed true-breeding plants that

had yellow, round seeds (YYRR) with true-breeding plants that had green, wrinkled seeds (yyrr).

Dihybrid Cross Crosses Involving Two Traits:

Color: Yellow, GreenShape: Round, Wrinkled

Yellow-RoundYellow-WrinkledGreen-RoundGreen-Wrinkled

The Y and R alleles and y and r alleles stay together

F1 offspring would produce yellow, round seeds.

The F2 offspring would produce two phenotypes in a 3:1 ratio, just like a monohybrid cross.

If the two pairs of alleles segregate independently of each other

Four classes of gametes (YR, Yr, yR, and yr) would be produced in equal amounts.

These combinations produce four distinct phenotypes in a 9:3:3:1 ratio

Practice problems

Cross a homozygous yellow, homozygous round plant with a green, wrinkled plant

Cross a homozygous yellow, homozygous round plant with a heterozygous yellow, wrinkled plant

Other Patterns of Inheritance

11.3

Variation in Inheritance

Incomplete DominanceCodominance (multiple alleles)

Polygenic inheritance

Incomplete Dominance Both alleles contribute to a

phenotype of a heterozygous individuals to produce a trait not like either parent.

Phenotype intermediate between two pure traits

Ex: Snapdragons, Andalusian chick.

Snapdragon Flower Color Alleles often written as capital letters with

superscriptsex. CRCR (red) x CWCW (white)

Incomplete Dominance revealed in Heterozygous Individual

A cross between a white-flowered plant and a red-flowered plant will produce all pink F1 offspring

Self-pollination of the F1 offspring produces 25% white, 25% red, and 50% pink offspring.

Codominance (multiple alleles)Two dominant alleles are

expressed at the same timeEx: Roan coat color in horses

CRCR, CWCW, CRCW

CRCR (Red)CRCR (Red)

CWCW (White)

CRCW (Roan)

If a roan cow (RW) is mated to a roan bull (RW), what are the phenotypes of the offspring?

Multiple Alleles Most genes have more than two alleles

in a population The ABO blood groups in humans are

determined by three alleles, IA, IB, and i.Both the IA and IB alleles are dominant to

the i allele The IA and IB alleles are codominant to each

other Because each individual carries two

alleles, there are six possible genotypes and four possible blood types.

ABO Blood Group System

Type A IA IA or IA i Type B IB IB or IB i Type AB IA IB Type O ii

Problems

A man homozygous for type A blood marries a woman who is heterozygous for type B blood. What are the possible genotypes and phenotypes of their children?

A couple has a child with type O blood. If one parent is type O, what are the possible genotypes of the other parent?

Polygenic inheritance When two or more genes effect a

single characteristicFor example, skin color in humans is

controlled by at least three different genes.

Imagine that each gene has two alleles, one light and one dark, that demonstrate incomplete dominance.

An AABBCC individual is dark and aabbcc is light

A cross between two AaBbCc individuals (intermediate skin shade) would produce offspring covering a wide range of shades

The range of phenotypes forms a normal distribution.

Importance of Environment

In some cases environment plays an important part in the expression of genes

Ex: Temperature (Siameses cat fur), Nutrition (height & growth of individual)

However human blood type is not influenced by environment.

Influence of Environment

Meiosis

11.4

MeiosisA type of cell division that

produces four cells Each cell hash half the number

of chromosomes as the parent cell.

In animals, meiosis occurs in the sex organs—the testes in males and the ovaries in females

Meiosis Each species has its own

number of chromosomes Humans 46 or 23 pairs Karyotype display of

these 46 chromosomes Homologus

Chromosomes chromosomes making up the pair that carry the genes for the same trait

Homologus Chromosomes

Two general typesAutosomes 22 pairs found both in

male & femaleSex Chromosomes carry the

gene that determines sex indicated by “X” or “Y”

Sex Chromosomes

Female 22 Autosomes + 1 Sex chromosome, XX 1 from father , 1 from motherEggs 1X + 22 Autosomes

MaleXY X from mother, Y from FatherSperm half have 1X+ 22 autosomes,

other half have 1Y+22 autosomes

Diploid and Haploid Cells

Diploid Cells Cells with 2 sets of chromosomes total number of chromosomes diploid # or 2n Humans 2n=46 in somatic cells

Haploid Cells 1n =23 chromosomes in sex cells, sperm & egg (Gametes)

Human Life cycle

Meiosis

Process by which haploid gametes are formed

Alternation of meiosis and fertilization keeps the # of chromosomes same from generation to generation.

Mitosis 2 offspring w/same # of chromosomes

Meiosis Gametes are formed egg & sperm 4 offspring with ½ the # of

chromosomes Exchange of genetic material

crossing over Oogenesis production of egg 1

egg + 3 smaller polar bodies Spermatogenesis production of

sperm 4 sperm cells

Two Distinct parts of Meiosis

Meiosis I

Meiosis II

Assortment of Chromosomes

Crossing Over/ Genetic Recombination

Comparison of Mitosis & Meiosis By three events in Meiosis l

1. Synapsis and crossing over Homologous chromosomes physically connect

and exchange genetic information

2. Separation of homologues At anaphase I of meiosis, homologous pairs

move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids

separate

3. Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous

chromosomes (tetrads) are positioned on the metaphase plates

Comparison of Mitosis & Meiosis