Genetics: The Science of Heredity

Lesson 1A Priest-ScientistGregor Mendel

Who was Gregor Mendel?• Gregor Mendel was an Austrian

monk.• He lived between 1822 to 1884.• He was a teacher & a botonist• He did experiments on hundreds

of pea plants.• Why Pea Plants?

– Simple genetic make up– Traits are easily observed– Can cross-pollinate or self-pollinate

Heredity• Father of Genetics

– Mendel’s work with pea plants has become the basis of genetics, the study of heredity.

• Heredity is the passing of traits from parents to offspring.

Mendel’s Pea Experiments

• He crossed plants with two different traits, for example purple flowers with white flowers.

• He started his experiments with purebred plants known as True Breeders.

• Purebred plants ALWAYS produce offspring with the same trait as the parent. For example, if the parent is tall, all offspring will be tall. If the parent is short, all offspring will be short.

Some Pea Traits that Mendel Studied

F1 Generation• Mendel called the parent plants the P generation.• He called the offspring from the parents the F1

generation. • F is from the Latin word, filial, which means son.• When Mendel crossed pure pea plants with purple

flowers with pure pea plants with white flowers, all the F1 generation had purple flowers.

P Generation (Parent Plants)

F1 Generation, Offspring of the Parent Plants

F2 Generation• When he crossed the F1 generation peas with

one another, only some of the offspring had purple flowers. These formed the F2 generation.

• Mendel found that in the F2 generation, ¾ of the plants had purple flowers and ¼ of them had white flowers (3:1 ratio).

F1 Generation, offspring of Parent Plants

F2 Generation, offspring of F1 Generation

Lesson 1 Review

1. Who was Gregory Mendel?2. Why did he choose peas for his

experiments?3. What is heredity?4. What is genetics?5. What is a purebred plants?6. What is the P generation?7. What is the F1 generation?8. What is the F2 generation?

Lesson 2

Understanding Mendel’s Experiments

Dominant and Recessive Traits• It seemed to Mendel,

that for each characteristic in peas, one trait was stronger than the other.

• He called the “stronger” one, the dominant trait.

• He called the “hidden” one, the recessive trait.

• This has become known as “Mendel’s Principle of Dominance”

Genes and Alleles• The traits of peas (and

yours) are controlled by factors that scientists call genes.

• You inherit your genes from your parents.

• The different forms of a gene are called alleles.

• You inherit a combination of two alleles from your parents.

Dominant and Recessive Traits in Peas

• For each of the 7 traits that Mendel studied in peas, there is a dominant allele and a recessive allele.

• If a plant inherits both a dominant allele and a recessive allele, the dominant allele masks the recessive allele.

Understanding Mendel’s Experiments

Part I

2 alleles for

purple

PP

1 allele for purple. 1 allele for white

Pp

2 alleles for white

pp

Understanding Mendel’s Experiment

Part II1 allele for purple 1 allele for white

2 alleles for purple

PP1 allele for purple 1 allele for white

Pp

2 alleles for white

pp

PpPp

Principle of Segregation

• Based on the outcome of his experiment, Mendel developed his “Principle of Segregation” that states that when forming sex cells, the paired alleles separate so that each egg or sperm only carries one form of the allele.

• The two forms of the allele come together again during fertilization.

Lesson 2 Review16. What did Mendel find to be the same with all 7

traits of the pea plant that he studied?17. What are genes?18. What are dominant alleles?19. What are recessive alleles?20.What happens if a pea plant inherits two

dominant allele of the same gene?21. What happens if a pea plant inherits a

dominant allele and a recessive allele of the same gene?

22. What happens if the pea plant inherits two recessive alleles of the same gene?

Lesson 3

Probability and Genetics

Probability• Probability is the likelihood

that a particular event will occur.

• The laws of probability determine what is likely to occur, not what does occur.

• Mendel was the first scientist that applied the principles of probability to genetics.

Punnett Square• Punnett square is a table that

shows all the possible combinations of alleles that can result when two organisms cross.

• Using Punnett square, geneticists can predict the probability of occurrence of a particular trait.

• The allele that each parent will pass to its offspring is based on chance, just like tossing a coin.

Genotypes and Phenotypes• Genotype: Indicates

the alleles that the organism has inherited regarding a particular trait.

• Phenotype: The actual visible trait of the organism.

Genotype

Phenotype

Homozygous and Heterozygous• Homozygous: An

organism with two identical alleles for a trait (a purebred organism).

• Heterozygous: An organism that has two different alleles for a trait (a hybrid organism).

Review

16. What it probability? How is it related to genetics?

17. What is the Punnett Square? How is it helpful to geneticist?

18. What is a genotype?19. What is a phenotype?20. What is a homozygous organism?21. What is a heterozygous organism?

Lesson 4

• Alleles

Genetics• Alleles

1. Alternative forms of genes.2. Homozygous alleles are exactly the same.

3. Dominant alleles – capitalized (TT - tall pea plants)

a. Homozygous dominant4. Recessive alleles - lowercase

(tt - dwarf pea plants)a. Homozygous recessive

5. Heterozygous (different) (Tt - tall pea plants)

Phenotype• Outward appearance• Physical characteristics

• Examples:1. tall pea plant2. dwarf pea plant

Genotype• Arrangement of genes that produces the

phenotype• Example:

1. tall pea plantTT = tall (homozygous dominant)

2. dwarf pea planttt = dwarf (homozygous

recessive)3. tall pea plant

Tt = tall (heterozygous)

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

1. What is the genotype of a homozygous tall plant?

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

2. What is the genotype of a homozygous short plant?

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

3. What is the genotype of a heterozygous tall plant?

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

4. A plant has a genotype of Tt. What is its phenotype?

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

5. A plant has a genotype of tt, what is its phenotype?

Practice! Practice! Practice!!!

In pea plants the Tall (T) allele is dominant over the dwarf (t) allele.

6. What are the two alleles for the height of a pea plant?

Answers:

1. TT2. tt3. Tt4. Tall5. short6. T (tall) and t (short)

Lesson 5Mono-Hybrid Crosses

Monohybrid Cross

• A breeding experiment that tracks the inheritance of a single trait.

• Mendel’s “principle of segregation”pairs of genes separate during gamete formation (meiosis) & reform during

ferlization.

Homologous Chromosomes

eye color locusb = blue eyes

eye color locusB = brown eyes

Paternal Maternal

This person would have brown eyes (Bb)

Monohybrid Cross

• Example: Cross between two heterozygotes for brown eyes (Bb)

BB = brown eyesBb = brown eyesbb = blue eyes

B

b

B b

Bb x Bb

malegametes

female gametes

Monohybrid Cross

BB

Bb

Bb

bb

B

b

B b

Bb x Bb

1/4 = BB - brown eyed1/2 = Bb - brown eyed1/4 = bb - blue eyed

1:2:1 genotype 3:1 phenotype

Practice! Practice! Practice!!!!

In cocker spaniels black (B) is dominant to red (rust) (b).

1. What would be the phenotypic ratio of a cross between a true breeding black crossed with a true-breeding rust?

Step 2 – List the possible gametes from each parent

BB bb

B B b b

Step 3 Draw punnett square and place the gametes on the sides.

BB bb

B B b b

B

B

b b

Step 4 Fill in the punnett square to find the possible zygotes

BB bb

B B b b

B

B

b b

B B

B Bb

b

b

b

Step 5 Determine the genotypic and Phenotypic ratios

BB bb

B B b b

B

B

b b

B B

B Bb

b

b

bPhenotype100% BlackGenotype 100% Bb

Practice ProblemsComplete a Punnett Square for each of

the following:T = tall plant t = short plantP = purple flowers p = white flowers

1. PP x pp 4. Pp x pp2. Tt x TT 5. tt x TT3. Pp x PP 6. Tt x tt

Lesson 6Dihybrid Cross

• A breeding experiment that tracks the inheritance of two traits.

• Mendel’s “principle of independent assortment”

a. during Metaphase I, each pair of alleles seperates independently

b. formula: 2n (n = # of heterozygotes)

Independent Assortment• Question: How many gametes will be produced

for the following allele arrangements?

• Remember: 2n (n = # of heterozygotes)

1. RrYy

2. AaBbCCDd

3. MmNnOoPPQQRrssTtQq

Answer:

1. RrYy: 2n = 22 = 4 gametesRY Ry rY ry

2. AaBbCCDd: 2n = 23 = 8 gametesABCD ABCd AbCD AbCdaBCD aBCd abCD abCD

3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes

Dihybrid Cross

• Example: cross between round and yellow heterozygous pea seeds.

R = roundr = wrinkledY = yellowy = green

RY Ry rY ry x RY Ry rY ry possible gametes produced

RrYy x RrYy

Dihybrid Cross

RY Ry rY ry

RY

Ry

rY

ry

Dihybrid Cross

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry

Lesson 7 Exceptions

Incomplete Dominance

• F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.

• Example: snapdragons (flower)• red (RR) x white (rr)

RR = red flowerrr = white flower

r

r

R R

Incomplete Dominance

Rr

Rr

Rr

Rr

r

r

R R

All Rr = pink(heterozygous pink)

produces theF1 generation

Codominance• In codominance, the

alleles are neither dominant, nor recessive. Neither allele is masked by the other.

Roan CowIs both white and red

Lesson 8

Mutations

MUTATIONS• Mistakes that occur during the duplication

of the chromatin material• GERM CELL MUTATIONS - occur in

gametes, passed on to offspring without affecting parent

• SOMATIC MUTATIONS - occur in body cells, not usually passed on to offspring

Mutations

• Chromosomal Mutations affect the entire chromosome

• Occur during the Crossing Over period of Meiosis– Duplication Mutations – one chromosome

carries two copies of a gene or set of genes– Deletion Mutations – post cell division, the

new cell will lack the genes that were carried by the piece that broke off

Chromosomal Mutations, cont.

– Inversion Mutations – the segment that breaks off reattaches itself but in the backwards position

– Translocation Mutations – a chromosome segment attaches itself to a non-homologous chromosome

Chromosome Mutations, cont.

NONDISJUNCTION-failure of a chromosome to separate from its homologue during meiosis

*one gamete will receive an extra copy of a chromosome while the other gamete will not have the chromosome

Results in: *DOWN SYNDROME (TRISOMY 21)-an extra copy of the 21st chromosome

Mutations

• Gene Mutations affect individual genes on a chromosome.

• Occur by: – Insertion of a nucleotide– or– Deletion of a nucleotide

The Number of Chromosomes

• Humans that have even one missing or one extra chromosome usually die before birth or have serious defects.

• Down Syndrome happens when a person is born with an extra copy of chromosome number 21.

Chromosomes• Males are XY and females are XX• Genes found on sex chromosomes…SEX

LINKED• Genes on an X chromosome are X-

LINKED. Genes on a Y chromosome are Y-LINKED.

• Eye color in fruit flies is X-Linked. Only male fruit flies have white eyes.

Sex linked traits

Examples of sex linked traits are1. Blood clotting factor – this factor is located

on the X chromosome and the dominant allele allows your blood to clot normally.

The recessive form does not allow your blood to clot.

Two recessive alleles causes the disease hemophilia.

Boys are more likely to get the disease because they only have one X.

2. Red-green color vision is another sex-linked trait. The dominant allele allows you to see reds and greens. The recessive allele prevents seeing red or green.Boys are most often affected because of having only 1 X chromosome.

Sex Influenced Traits• Influenced by male or female sex

hormones• Baldness

Multiple Alleles

• Traits that are determined by 2 or more alleles & produce different colorations based on the original traits.

• Include: – blood type– Hair color– Eye color

Polygenic Traits

• The results of the interaction of multiple genes

• Example: hypertension– Caused by the interaction of one’s:

• Weight• Ability to process fats• Cholesterol count• Ability to process salts• Lifestyle - i.e. Smoking, drinking, exercise

Mutagens

• Environmental factors that can cause mutations

• Include: – Radiation exposure via X-rays or UV light– Natural or made-made chemicals– Pollutants– Extremely high temperatures– Viruses

Human Karyotype showinghomologous chromosome pairs

This individual has inherited three copies of chromosome 21 and has a condition called Down syndrome.

Nondisjunction, cont.• KLINEFELTER’S SYNDROME- (XXY)-

feminine characteristics, mentally impaired, infertile

• TURNER’S SYNDROME- (XO)- female appearance, no sexual maturity, infertile

Human Genetic Disorders

• Cystic fibrosis-(CF)-difficulties with breathing and digestion

• Sickle cell anemia -forms sickle shaped RBCs because of a defective protein called hemoglobin, leads to lack of O2 & circulatory problems

Fig. 11.12, p. 183