Bio Unit 7 Notes pp.pptx

BIOLOGY

Unit 7

Mendelian Genetics

A. History of Genetics

1. Genetics – the study of genes and heredity 2. Inheritance – the process by which

characteristics of individuals are passed to their offspring

3. Genes – part of chromosome that codes for a

trait

a. Locus (loci) – the gene’s physical location on the chromosome

b. Both pairs of homologous chromosomes carry the same genes located at the same loci

Mendelian Genetics

4.

Analogy

a. Alphabet = nucleotide bases A T C G b. Words = triplet codes CAT TAG

c. Sentences = genes the CAT plays TAG d. Paragraph = DNA

Mendelian Genetics

5.

Gregor Mendel

a. In 1843, at age 21, he entered a monastery in

Austria

b. He was asked to take care of the garden – this

got him interested in science

c. In 1851, he was sent to the University of

Vienna to study science and math

d. When he returned, he started studying

Mendelian Genetics

B.

Mendel’s Experiments

1.

He looked at 7 pea plant characteristics:

each variant is a trait

a. Seed texture – round or wrinkled b. Seed color – yellow or green

c. Seed coat color – colored or white

d. Pod appearance – inflated or constricted e. Pod color – green or yellow

f. Position of flowers on stem – axial or

terminal

Mendelian Genetics

2.

Why study pea plants?

a. Each trait only has 2 characteristics b. A lot easier than studying a human

3.

He looked for predictable patterns in the

inheritance of each trait

4.

He performed thousands of tests over 9

years

a. Step 1:

1. Grew “pure” plants – plants that are pure

for 1 characteristic

2. Example – 1 pea plant would always have

green seeds

3. He produced 14 (7x2) groups of pure plants

– parental generation (P₁ generation)

Mendelian Genetics

b. Step 2:

1. He began crossing pure plants of each

characteristic for the same trait

a. Called hybridization

2. Example – crossed green pod plant with a

yellow pod plant

3. He called the offspring – First Filial

Generation (F₁ Generation)

4. Results:

a. All F1 offspring had green pods

b. Only one of the traits from the G1 generation

Mendelian Genetics

c. Step 3:

1. Then he self-pollinated F₁ generation plants 2. He called these new offspring the F₂

generation

3. Results:

a. When those plants grew, he found that ¾

(75%) had green pods and ¼ (25%) had yellow pods

b. The trait that had “disappeared” during the

Mendelian Genetics

C. Mendel’s Conclusion

1. The Principle of Dominance and Recessiveness

a. The trait that appeared in the F₁ generation – Dominant Trait

b. The trait that appeared in the F₂ generation – Recessive Trait

c. “One factor in a pair may mask the other factor, preventing it from having an effect.”

2. The Principle of Segregation

a. If each parent has 2 factors, then each offspring will

Mendelian Genetics

3. The Principle of Independent Assortment

a. The factors of different characteristics did

not affect one another

b. Example – just because a plant produced

green seeds doesn’t mean that the plant has to have wrinkled seeds

c. The traits are inherited independently from

Mendelian Genetics

1. He presented his work to the scientific world in a

paper in 1865

2. Because scientists had no knowledge of meiosis,

chromosomes, genetics, they did not accept nor understand his work

3. He was before his time!

4. It wasn’t until 1900 when 3 scientists rediscovered,

understood, and accepted his work

5. Mendel’s factors were given a name – allele –

alternate versions of a gene

a. Dominant allele – capital letter

Patterns of Inheritance

A.

Genotype and Phenotype

1. Genotype

a. Genetic makeup or an organism b. Example – for a pure tall plant: TT

2. Phenotype

a. External, physical appearance of an

organism

Patterns of Inheritance

3. Dominant and recessive alleles

a. Dominant allele – capital letter (T)

b. Recessive allele – lower case letter (t)

c. For an organism to express the dominant

trait, it only needs 1 dominant allele

1. Tall = TT or Tt

d. For an organism to express the recessive

trait, it needs BOTH recessive alleles

1. Short = tt

Genotype Tt tt TT

Patterns of Inheritance

B.

Homozygous vs. Heterozygous

1. Homozygous

a. When both alleles of a pair are the same b. 2 types:

1. Homozygous dominant – TT 2. Homozygous recessive – tt

2. Heterozygous

Patterns of Inheritance

C. Monohybrid Crosses

1. A cross between individuals that involves 1

pair of contrasting traits

2. Use Punnett Squares

a. Predict probabilities of genetic crosses

3. Example 1 – Homozygous Dominant x

Homozygous Recessive (TT x tt) T T

Patterns of Inheritance

4. Example 2: Homozygous Dominant x Heterozygous (TT x Tt)

T T

T

t

Patterns of Inheritance

5. Example 3: Heterozygous x Heterozygous (Tt x Tt)

T t

T

t

Patterns of Inheritance

6. Example 4: Homozygous Recessive x Heterozygous (tt x Tt)

t t

T

t

Genotype: 2Tt : 2 tt

Phenotype: 2 tall, 2 short

Patterns of Inheritance

Testcross

1. If you produce a rabbit with a black coat,

how do you know if it is a homozygous dominant (BB) or heterozygous (Bb) for a black coat if B is black and b is white?

2. So, you cross this individual with an

unknown genotype with a homozygous recessive one

3. Do Punnett squares to predict the outcomes

of this cross:B B

b

b

B b

b

Patterns of Inheritance

4. If all the offspring still have black coats, then

parent must have been BB

5. If any offspring have a white coat, then the

Patterns of Inheritance

E.

Dihybrid Cross

1. A cross between individuals that involves 2

pairs of contrasting traits

2. Steps:

a. Start with the 2 traits (for example – height and

seed texture)

1. TtRr (T = tall, t = short, R – round, r = wrinkled)

b. Use FOIL method to get combinations

Patterns of Inheritance

c. Put these 4 combinations along the top and

side of a Punnett square

d. Fill in the Punnett square

1. **keep all the Ts together and Rs together 2. **Put capital letter first

TR Tr tR tr TR

Tr tR tr

Patterns of Inheritance

Sex chromosomes vs. Autosomes

1. Sex chromosomes determine male or female

gender

a. XX is female b. XY is male

1. Sperm cells can carry either an X or a Y, so sperm

essentially determine the sex of the offspring

Patterns of Inheritance

3. Sex-linked genes

a. Genes that code for traits that are found on the sex chromosomes

b. Because females have 2 Xs, they can be

homozygous or heterozygous for traits found on their X chromosome

1. This is why females can be “carriers” for some diseases or have the disease

c. Because males have only 1 X chromosome, whatever trait their X has on it is what will be expressed

4. Example: hemophilia – blood disorder where the

Patterns of Inheritance

G. Other genetic Conditions

1. Incomplete dominance – sometimes the expression

of two different alleles in a heterozygous condition produce a “blending” effect

2. Codominance – when both alleles of a

heterozygous condition are expressed

Pedigrees

A.

Pedigrees

1. Graphic representations of how traits

are passed through a family from generation to generation

2. Steps to figure out a pedigree: