Mendelian Genetics

Blue People of Kentucky

Methemoglobinemia

Genetics

• Genetics is the study of heredity.

• Heredity is the transmission of traits from parent to offspring; an organisms genetic makeup.

• Genes are sections of a chromosome that code for a specific trait. Ex: hair color, height, etc.

• A trait is any characteristic that can be passed from parent to offspring.

Genetics

• Alleles are the form of the gene that is passed from parent to offspring. • Flower color = gene;

• Purple and white = alleles of the flower color gene

• Multiple alleles are genes with more than 2 alleles• Eye color

Gregor Mendel• An Austrian monk who is credited as the first person to

study the science of genetics.

• So he is referred to as “the Father of Genetics.”

• Mendel worked with garden pea plants in his experiments• Easy to grow

• Had many traits to study; height, seed shape, pod shape, flower color, etc.

• Reproduced rapidly

• Cross pollination

Gregor Mendel

• Mendel’s Experiment• Crossed a Tall plant with a short plant

• This is the parent generation, P1

• All offspring were tall

• Crossed two plants from the P1 offspring

• This is the first filial or first generation of offspring, F1

• Crossed Tall plants from the next generation

• This is the second filial generation of offspring after the parents, F2

• Resulted in ¾ Tall plants and ¼ short plants

Mendel’s Findings

• Concept of Unit Characters: traits are determined by “factors” that occur in pairs. We now know that these unknown “factors” are genes.

• Dominance: One factor of a pair may cover up or prevent the expression of a trait

Example: Your Mom gives you the gene for having a Unibrow (recessive) and your father gives you the gene for having two eye brows (dominant)

Mom Dad

What will you have?

Mendel’s Findings

• Law of Segregation: two alleles for a gene will separate during meiosis

• Law of Independent Assortment: alleles separate randomly or independently of each other. This means that seed shape does not affect plant height.

Because of independent assortment and crossing over during meiosis, there is an infinite number of possible combinations of chromosomes in the gametes, this is called genetic recombination.

Important Genetics Terms

• Dominant: the “stronger” gene; represented by an uppercase letter

• Recessive: the “weaker” gene; represented by a lowercase letter

B R

b r

Important Genetics Terms

• Genotype: combination of genes represented by a combination of letters. We generally use the first letter of the dominant gene.

• RR, Rr, rr

• Phenotype: the expression of the gene/trait; what is actually seen or expressed (phenotype = physical trait)

• Tall, short, green, round, wrinkled

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Genotype & Phenotype

Genotype of alleles:R = red flowerr = yellow flower

All genes occur in pairs, so 2 alleles affect a characteristic

Possible combinations are:Genotypes: RR Rr rr

Phenotypes: RED RED YELLOW

Important Genetics Terms

• Homozygous: gene pairs that are identical• Homozygous dominant: TT, AA, RR

• Homozygous recessive: tt, aa, rr

• Heterozygous: gene pairs that are different. The dominant gene will be expressed in most heterozygous organisms.

• Tt, Aa, Rr

• Hybrid: offspring of parents with gene pairs that are different; another term for heterozygous

Probability

• The traits you received from your parents were all randomly combined due to chance.

• We use the mathematical study of probability in genetics to help us determine the likelihood of passing on certain traits to future offspring.

Mendelian Inheritance and Probability

• How do we know what the probability of a cross will be?

• We use a device called a Punnett square to see possible gene combinations.

Mendelian Inheritance and Probability

• Monohybrid Cross: a cross between organisms that involves one trait.

• Dihybrid Cross: a cross between organisms that involves two traits

Mendelian Inheritance and Probability

• The probability of a gene combination can be represented by: • A fraction: # of one kind of combination

# of all possible combinations

• A ratio: 1:4, 1:2, 3:4, 4:0, 1:2:1, etc.

• Or a percentage: 25%, 50%, ect.

Let’s practice… Punnett squares

WHEN HEREDITY ISN’T SIMPLE

INCOMPLETE DOMINANCE• The phenotype of the heterozygous organism is “in

between” the dominant and recessive phenotype.

• Example: Snapdragon flowers• Homozygous Dominant = Red (CRCR)

• Homozygous Recessive = White (CWCW)

• Heterozygous = Pink (CRCW)

CODOMINANCE• Both alleles express themselves independently in the

phenotype. This means that both alleles are expressed equally; both are dominant in a heterozygous organism.

• Cows, horses, people (sickle cell anemia)• Homozygous dominant = Black (BB)

• Homozygous dominant = White (WW)

• Heterozygous = Checkered (BW)

Multiple Alleles

• Traits controlled by more than two alleles.

• Blood Type

Sex-Linked Traits

• Traits carried on the sex chromosome

• Sex-linked traits can only be carried on an X chromosome.

• Color Blindness

• Male Pattern Baldness

• Can a father pass a sex-linked trait to his son?

Autosomal Disorders

• Nondisjunction

• Monosomy• Turner Syndrome

• Trisomy• Trisomy 21 = Down’s Syndrome

Sex Chromosome Disorders

• Turner’s Syndrome

• Kleinfelter’s Syndrome

• Superfemale

Dominant Disorders

• Achondroplasia (dwarfism)

• Glaucoma

• Polydactyly

• Brown Teeth

Recessive Disorders

• Cystic Fibrosis

• Sickle Cell Anemia

• Albinism

Pedigrees

• Shows the pattern of inheritance within a family