Genetics

Genetics

Explain the basic rules and processes associated with the

transmission of genetic characteristics

Describe the evidence for dominance, segregation and

the independent assortment of genes on different

chromosomes, as investigated by Mendel

Introduction• Heredity

– is the passing of traits from parents to offspring• Genetics

– is the study of the patterns of inheritance as hereditary characteristics or traits

• Gregor Mendel – the "father of modern genetics“– an Austrian monk – published his completely new and thoroughly

documented ideas of inheritance in 1866

Introduction

• His model was so simple that scientists who read it at that time considered it "trivial“– it received little attention and no recognition until it

was rediscovered in 1900 after his death (simultaneously by 3 different people)

• In the meantime, – chromosomes had been named – their movements during mitosis and meiosis

observed and described

Introduction

• 1902 – scientists realized that chromosomes moved

precisely as reported by Mendel• Once the connection between chromosomes

and heredity was established, the science of genetics was reborn

• Mendel was the first person to realize that genetic traits are inherited as separate particles

Introduction• He did not actually see these particles

– but he predicted their existence based on patterns of inheritance

• He proposed that organisms have a pair of "factors" for each trait– one from each parent

• We NOW know that the particles of inheritance are segments of DNA– which we call genes

Mendel's Experiments • Mendel worked with garden peas

– available in many different varieties • E.g. pure breeding tall, pure breeding dwarf

• Pea flowers contain both male and female parts and normally self-pollinate– but can easily be artificially cross-pollinated with

other pea plants• Mendel crossed plants of two varieties with

contrasting traits (such as tall and dwarf) to see what would happen.

Mendel's Experiments• Mendel worked with seven traits, each which

occurred in two distinct forms• Mendel began by studying crosses involving

only one trait at a time• Example: Flower color

– P1: (parental generation) • pure-breeding red flowered plants X pure-

breeding white flowered plants– F1: (first filial generation)

• red-flowered hybrids (genetically mixed offspring)

– F2: (second filial generation)• 3/4 red-flowered and 1/4 white-flowered

Mendel's Laws

1. Inherited characteristics are controlled by pairs of factors – genes– one from each parent.

2. One gene may “mask” the effect of another. – The gene which is expressed is dominant, while

the one which is masked is recessive.3. Pairs of genes segregate during gamete

formation– so each sex cell contains only one member of a

pair of genes.

Terms • Alleles

– Two or more alternate forms of a gene, which produce contrasting effects for a certain trait

– e.g. red (R) and white (r) for flower colour of peas• Homozygous

– having two of the same allele• eg. red/red (RR)• purebred

• Heterozygous – having two different alleles eg. red/white (Rr)

Terms• Genotype

– the genetic makeup of an individual• Phenotype

– the expression of the genes, or appearance of an individual

• Purebred – an organism having all homozygous gene pairs

• Hybrid – an organism having at least one heterozygous

gene pair

Terms

• Monohybrid – an organism having only one

heterozygous gene pair• Dihybrid

– an organism having two heterozygous gene pairs

Monohybrid Cross

Monohybrid Punnet Square

RrRrr

RrRrr

RRMale Female

Monohybrid Cross

Monohybrid Cross

Monohybrid Cross

• The phenotypic ratio for a monohybrid cross is always 3:1 –dominant trait : recessive trait

Predicting the Outcome of a Genetic Cross

• When we know the genotypes of parents used in a genetic cross, we can predict the genotypes of the offspring and their expected ratios– Must use a Punnett square is used

• named after some guy named Punnett• A geneticist

– Basically it’s a just a fancy chart

Monohybrid Punnet Square

RrRrr

RrRrr

RRMale Female

Practice

If an organism has the dominant phenotype, how can

you determine whether it is homozygous or heterozygous?

Test Cross

• You conduct a test cross.– Mate it with an organism of a known

genotype and see what you get.• The only known genotype that is observable is

HOMOZYGOUS RECESSIVE (rr)• For example:

– determine whether a red-flowered pea plant is homozygous or heterozygous

Test Cross

P: Red flowered X White-flowered  R? rrF1: Suppose the cross produces all red flowers  Rr Rr Rr Rr• If no offspring showing the recessive

phenotype are produced, the unknown parent must be …– homozygous

Test Cross

P: Red flowered X White-flowered  R? rrF1: Suppose the cross produces 50% red flowers and

50% white flowers • The only way a white flower could appear is if it

received a recessive allele from the unknown parent. – The unknown parent must be …– HETEROZYGOUS

Dihybrid Cross

• In addition to his monohybrid crosses, Mendel performed dihybrid crosses of plants with two different pairs of contrasting alleles

• In one experiment, Mendel crossed plants homozygous for seeds that were both round and yellow with plants homozygous for wrinkled, green seeds

Dihybrid Cross

• All the F1 offspring had round, yellow seeds• Self-fertilization of the F1 plants produced

and F2 generation of seeds with the following phenotypes:

315 round yellow108 round green101 wrinkled yellow 32 wrinkled green

Dihybrid Cross

• To find the ratio among the F2 phenotypes, – we take the number of offspring in the

smallest category - 32 - and divide it into the number of offspring in the other categories:

– then the quotient is rounded to the nearest whole number

Dihybrid Cross

• Thus Mendel determined the phenotypic ratio in the F2 generation to be 9:3:3:1– This is the ratio typical of a dihybrid cross

in which both pairs of alleles show a dominant-recessive relationship

Dihybrid Cross • Mendel explained these data by assuming that the

genes governing seed color and seed shape move independently during gamete formation

• In the process of independent assortment, each pair of alleles behaves as it would in a monohybrid cross - independently of the other pair 

• A dihybrid can produce four possible gene combinations (with equal probability)

• If the alleles are:R = round r = wrinkledY = yellow y = green

• The possible gamete combinations are RY Ry rY ry

Dihybrid CrossRRYY x rryy RY ry

RrYy RY, Ry, rY, ry

9/16 round, yellow (R_Y_)3/16 round, green (R_yy)3/16 wrinkled, yellow (rrY_)1/16 wrinkled, green (rryy)

Parent:P Gametes:

F1:F1 Gametes:

F2:

Practice

Multiple Alleles&

Incomplete Dominance

• Compare ratios and probabilities of genotypes and phenotypes for dominant and recessive, multiple, incomplete dominant, and codominant alleles

• Explain the relationship between variability and the number of genes controlling a trait

Multiple Alleles• The genes which we have studied so far

have only two different alleles: • Many genes actually exist in more than two

allelic forms, • Although only two genes control coat colour

in rabbits it is controlled by a series of four alleles for the same gene:

1. C Full colour2. Cch Chinchilla3. ch Himalayan4. c Albino

C + any of the 4 Cch+ anything but C

ch ch, ch cc c

Multiple Alleles• The dominance hierarchy of these

alleles is C > Cch > ch > c• Determine the genotypes for the

following phenotypes:Phenotype Possible Genotype1. Full color2. Chinchilla3. Himalayan4. Albino

Incomplete Dominance

• In some cases, a heterozygous organism shows a blending of genes because neither gene is dominant: – this is termed incomplete dominance – for example, in snapdragons, neither the

red nor the white allele is dominant…

Incomplete Dominance

Red Flowers x White Flowers 

Pink Flowers 

¼ Red flowers ½ Pink flowers¼ White flowers

Parent:

F1:

F2:

RR x rr

Rr, Rr, Rr, Rr

RRRrrr

Codominance• If two different alleles each contribute to a

phenotype, they are termed codominant• One of the best-known example of

codominant genes occurs in humans, and determine ABO blood types

• There are three possible alleles: 1. IA

2. IB

3. i

CodominanceBlood Type

i iO

IA IBAB

IB IB, IB iB

IA IA, IA iA

Possible GenotypesPhenotype

Practice

Chromosome Mapping

• Explain the influence of gene linkage and crossing over on variability

Chromosome Mapping

Chromosomal Theory of Inheritance

1. Genes are located on chromosomes 2. Chromosomes undergo segregation

during meiosis 3. Chromosomes assort independently

during meiosis 4. Each chromosome contains many

different genes

Chromosome Mapping

• Each chromosome contains hundreds or thousands of genes

• Genes located on the same chromosome are inherited together, – they are part of a single chromosome

that is passed along as a unit – such genes are said to be linked

Chromosome Mapping• during meiosis, chromosomes may exchange

segments of DNA by crossing-over

A A a aB B b bC C c c

D D d dE E e eF F f f

Chromosome Mapping• during meiosis, chromosomes may exchange

segments of DNA by crossing-over

A A a a A a A aB B b b B B b bC C c c C C c c

D D d d D D d dE E e e E e E eF F f f F f F f

Chromosome Mapping• The closer two genes are on a chromosome,

the fewer the possible points of crossover are between them, and the less frequently such a cross-over will occur

• In other words, if two genes are close together on a chromosome it is likely they will stay together and not be exchanged between chromatids during meiosis

• To determine the location of genes along a chromosome is called MAPPING a chromosome

Chromosome Mapping

• A chromosome map indicates 1. The order in which specific

genes occur on a chromosome2. The distances between the

genes

Chromosome Mapping• Example:

– In Drosophila, the following data was obtained from genetic crosses: • 13% recombination between bar eye and garnet eye

– High percentage recombination indicates that these two genes are far apart from each other

– High likelihood that crossing over will occur between these two genes.

• 7% recombination between garnet eye and scalloped wings

– These two genes are closer together than bar eye and garnet eye

• 6% recombination between scalloped wings and bar eye

Chromosome Mapping

• This data can be used to map the chromosome:

 Bar eye Scalloped wings Garnet eye 

6 units 7 units

13 units

Chromosome Mapping

Answer questions to the Lab Exercise on page 639 of your

Nelson textbook. Hand in when done.

Sex Determination• There are two chromosomes involved in the

determination of sex of most animals,– the sex chromosomes (X & Y)

• Any other chromosome not involved in sex determination is called an autosome– for example, humans have 22 pairs of autosomes

and 1 pair of sex chromosomes • in most mammals, females are homozygous

and have 2 X chromosomes, while males are heterozygous and carry an X and a Y chromosome

Sex Determination

• Female = XX• Male = XY • Females can produce eggs carrying

only an X chromosome, • Males produce sperm carrying either an

X or a Y chromosome (50% of each)• Thus, it is the male who determines the

sex of his offspring !

Sex Linkage

• Compare the pattern of inheritance produced by genes on the sex chromosomes to that produced by genes on autosomes, as investigated by Morgan and others.

A True Story• One day a geneticist named Thomas Hunt

Morgan discovered a mutant white-eyed male fly among his hundreds of red-eyed flies. He bred the white-eyed male with several red-eyed females and observed the offspring: as expected, all the F1 flies had red eyes, showing red to be the dominant allele. He allowed the F1 generation to interbreed freely and observed the F2 generation. Again, as expected, the ratio of red-eyed flies to white-eyed flies was 3:1, except that …

every white-eyed fly was male!

Explanation …

Morgan concluded that the gene controlling eye-colour was carried

on the… X-chromosome

Sex Linkage

• Like other chromosomes, the sex chromosomes carry many genes

• Some of the regions of the X-chromosome have a homologous region on the Y- chromosome– There are also large non-homologous

portions: • That is, the X chromosome carries

some genes that have no counterparts on the Y chromosome

Sex Linkage

• Genes for sex-linked traits are carried on the X chromosome but not on the Y chromosome– Therefore in a male, the gene on the X

chromosome is expressed whether it is dominant or recessive

– In a female, she must have two recessive alleles to have the recessive phenotype

Fly Solution

• R = Red• r = white• XR XR = Red eyed Female• XR Xr = Red eyed Female• Xr Xr = White eyed Female• XR Y = Red eyed Male• Xr Y = White eyed Male

Fly Solution

XR XR x Xr Y

XR Xr x XR Y

XR YXR YY

XR XrXR XrXr

XRXR

Xr YXR YY

XR XrXR XRXR

XrXR

Example of hemophilia trait – (h) recessive disease

Sex Linkage

• Some sex-linked traits in humans are – Colour vision– Hemophilia– Duchenne's Muscular Dystrophy

Sex-Influenced Genes

Sex Influenced Genes• The main role of sex hormones is to

influence the reproductive system and its related organs, – These hormones, however, also affect

many other parts of the body• Genes that are expressed to a

greater or lesser degree as a result of the level of sex hormones are called sex-influenced genes.

Sex Influenced Genes• These genes are usually located on the

autosomes• Males and females with the same

genotype may differ greatly in phenotype because the levels of sex hormones

• For example: – A bull may have a gene for high milk

production, but he will notproduce milk because he has low levels of female hormones.

Sex Influenced Genes• In humans, the gene for male pattern

baldness is autosomal and sex-influenced • A man will become bald even if he has

only one allele for baldness, because– the male sex hormones somehow stimulate

the expression of the allele• In a woman, however, the allele acts as a

recessive allele so that she must have two balding genes before she loses her hair

Lethal Alleles

Lethal Alleles• If An organism has a mutation that destroys

the genetic code for a protein essential to life, the organism will often die prematurely.

• This gene that fails to code for a functional protein is called a lethal allele

• It is possible for lethal alleles to be dominant, but most are rapidly eliminated from a population because they cause death before the individual carrying the allele reproduces.

Lethal Alleles• An exception of a lethal dominant allele that

remains in a population is the one responsible for Huntington's Disease in humans, because this allele is not expressed until later in life (35 - 45 years of age)

• An example of a recessive lethal allele in humans is the one for Brachydactyly:– Heterozygotes have a short middle finger bone

that makes the fingers appear to have only two bones instead of three

– Homozygous babies lack fingers and have abnormal development of the skeleton that result in death in infancy

Lethal Alleles

• Some human examples of lethal alleles are:– Sickle cell anemia– Tay-sachs disease– Cystic fibrosis– Huntington's disease

Pedigrees

Pedigrees

Pedigrees• Because geneticists are unable to manipulate

the mating patterns of people, they must analyze the results of matings that have already occurred.

• As much information as possible is collected about a family's history for a particular trait, and this information is assembled into a family tree describing the interrelationships of parents and children across the generations – This is called a pedigree

Pedigrees• From a pedigree you should be able to

determine if a particular trait is dominant or recessive, sex-linked or autosomal

• A pedigree not only helps us understand the past but also helps us predict the future

• Geneticists, physicians, and genetic counselors use pedigrees– for analysis of genetic disorders – to advise prospective parents of genetic risks

involved  • Complete the pedigree studies on page 632

and page 611 as a class.