GeneticsMendel’s Laws of Heredity

GeneticsMain Concepts…

a. Sexual reproduction involves the random distribution of genes.

b. We have thousands of genes.c. Genes determine your traits.d. Genes line up on your chromosomes.e. Chromosomes are in the nucleus of the cell.f. Chromosomes duplicate and divide in cell

division.g. Chromosomes are typically in pairs.h. A diploid cell has complete set of chromosomes.

GeneticsFrog: 13 pairs

Pea: 7 pairs

Apple: 17 pairs

Chimpanzee: 24 pairs

Dog: 39 pairs

1. HEREDITY – the passing of characteristics from parents to offspring

2. Characteristics that are inherited are called TRAITS

Heredity

1. Gregor Mendel was an Austrian monk and was the first person to successfully study GENETICS, the branch of biology that studies heredity

Gregor Mendel

Pea Plants

1. Mendel chose to study the garden pea plant:

a) The male gamete is pollen and fuses with the female gamete in a process called FERTILIZATION

i. Fertilized egg = zygote

b) The transfer of male pollen to the female reproductive organ is called POLLINATION

1. Pea plants can reproduce by SELF-POLLINATION because each plant has both male and female gametes

2. Mendel performed a process called CROSS-POLLINATION by dusting pollen from one plant to any other plant of his choosing. This allowed him to control which plants mate with which plants

Pea Plant (con’t)

Research

1. Mendel was a careful researcher:

a) He studied only one TRAIT at a time (e.g. Height) and analyzed the data mathematically

b) Mendel used TRUE BREEDING plants for his crosses. This means the plants have the same traits for many generations (e.g. all purple flowers)

i. Pure bred

1. When Mendel crossed two different ALLELES for a trait he called this a HYBRID

2. When Mendel only crossed one trait at a time he called this a MONOHYBRID CROSS

Experiment

1. All pea plants are either tall (6ft) or short (2ft)

2. The 1st Generation:a) Mendel cross-pollinated a true breeding tall

pea plant with a true breeding short pea plant. The offspring were all tall pea plants

b) P1 Tall x short = All tall plants

Monohybrid Cross (Height)

Monohybrid Cross (Height)

1. The 2nd Generation:

a) Mendel allowed the tall offspring to self-pollinate.

b) F1 Tall x Tall = 75% Tall/25% Short

c) 3:1 ratio of tall

Mendel’s Experiments ReviewShort pea plants X Tall pea plantsQ: What did he get?A: All Tall pea plants!!!!

Then, Tall X Tall pea plantsQ: What did he get?A: 787 Tall pea plants & 277 short pea plants

3:1 Ratio = 75% tall : 25% short

Generations

1. The original (true breeding) parents are known as the P1 generation

a) P = “parent”

2. The offspring of the P1 generation are known as the F1 generation

a) F = “filial” (son or daughter)

3. Crossing two F1 plants creates the F2 generation

Conclusions from Mendel’s Experiments1. Factors = Genes2. Law of Dominance: in any pair of genes, one

may hide the appearance of the other.a) Dominant: expressedb) Recessive: masked, hidden, not expressed

3. Law of Segregation: During Meiosis chromosomes separate, and genes separate also.

4. Law of Independent Assortment: the pairs of genes (and chromosomes) separate randomly!

Rule of Dominance

1. Mendel also concluded that one allele is DOMINANT and one is RECESSIVE

2. The dominant allele is the one that shows up in the F1 generation (Tall)

a) Capital letter T

3. The recessive allele is the trait that is hidden in the F1 generation (Short)

a) Lowercase letter t

4. Capital letter is always written before the lowercase letterTt tT

1. A dominant trait (tall) is the result of either TT or Tta) TT = tall b) Tt = tall

2. A recessive trait (short) can ONLY be the result of tta) tt = short

3. The dominant allele always overrides the recessive allele, therefore, dominant traits are more common than recessive traits

Dominant/Recessive Traits

Purple is dominant White is recessive

Purple is present(White is hidden)

AA aa

Aa

AaAaAA aa

Alleles

1. Mendel concluded that each organism must have two factors that control each of its traits

2. We now know that traits are controlled by GENES and are located on chromosomes.

3. Genes exist in alternative forms called ALLELESa) Gene plant height

i. Alleles tall or shortii. Alleles dominant or recessive

1. Alleles are located in exactly the same position on homologous chromosomes

2. In a diploid organism, there are two alleles for a given gene

a) One from mother; one from father

3. A plant might have:a) 2 copies of the tall allele (TT)b) 2 copies of the short allele (tt)c) 1 tall and 1 short allele (Tt)

Alleles

♂

♀

Law of Segregation

1. The Law of Segregation – every individual has two alleles for each gene and each gamete receives one of these alleles

Law of Segregation

Each gamete only gets one allele

Each offspring has one allele from each parent

The Law of Independent Assortment

1. The Law of Independent Assortment – genes for different traits are inherited independently of each other

a) Height does not affect flower color; they are independent of each other

b) Independent assortment results in increased genetic diversity because of the shuffling of individual genes

Law of Independent Assortment

1. PHENOTYPE: physical appearance; what it looks like.

2. GENOTYPE: genetic make-up; combination of genes.

3. Represented by 2 letters.4. 3 possible genotypes.

EX. Green pea pod, pink flower, tall stems

EX. GG, Gg, gg

Genotype vs. Phenotype

Homozygous vs. Heterozygous1. Homozygous – two alleles are the SAME

a) Homozygous dominant TTb) Homozygous recessive ttc) True breeding (pure bred)

2. Heterozygous – two alleles are DIFFERENTa) Ttb) Hybrids

Traits due to 1 gene

1. Dominant2. Free earlobe3. Left handedness4. Tongue rolling5. Left arm on top6. Left thumb on top

1. Recessive2. Cleft chin3. Dimples4. Attached earlobe5. Second toe longer6. Widow’s peak7. Double jointedness

Human Genetics

Dimples

Cleft Chin

Widow’s Peak

Free and Attached Ear lobes

Second toe is longer than big toe

Tongue rolling

1. Reginald Punnett devised an easy system to predict genotypic outcomes of a cross called a PUNNETT SQUARE

2. Parents gametes are combined in every possible combination

3. Monohybrid cross – one gene (height)4. Dihybrid cross – two genes (height and color)

Punnett Squares

Father’s Gametes (sperm)

Moth

er’s

Gam

ete

s

(eg

g)

PossibleOffspring

#1

PossibleOffspring

#2

PossibleOffspring

#3

PossibleOffspring

#4

Monohybrid Cross

TT x ttSetting up the Punnett square

TT tt(Short)(Tall)

Mom Dad

T = tallt = short

T

T

x

t t

Solving the Punnett Square

T

t

T

tt

T

t

ttTT

T

(Tall) (Short)

Tall Tall

Tall Tall

(Heterozygous)

(Heterozygous) (Heterozygous)

(Heterozygous)

T = tallt = short

Mom Dad

TTt x TtSetting up the Punnett square

Tt t(Tall)(Tall)

TT

T

t

x

T t

Solving the Punnett Square

TT

t

t

t

TtTt

t

(Tall) (Tall)

Tall Tall

Tall Short

(Homozygous)

(Homozygous)(Heterozygous)

(Heterozygous)

1. Offspring outcomes can be expressed as:

a) Fractions – ¼ or ¾

b) Percent – 25% or 50% or 75%

c) Ratio – 3:1 or 1:2:1

Fractions, Percent, Ratios

Test Cross

1. A TEST CROSS is a cross of an individual of unknown genotype with an individual with a known genotype

a) Test crosses use homozygous recessive individuals because the phenotype always indicates the genotype (short = tt)

2. Example:a) Tall pea plant: genotype unknown (TT or Tt)b) Short pea plant: known genotype (tt)c) Cross the tall plant with the homozygous recessive and look at the

offspringi. If the offspring are all tall then the unknown genotype is TTii. If the offspring are 50% tall and 50% the unknown genotype is

Tt

Tall Plant(TT or Tt?)

Short Plant(tt)

P1

Test Cross

F1… TT TtOR