5/23/2017

1

H E R E D I T Y MENDEL AND THE GENE HYPOTHESIS• Gregor Mendel (1822-1884): the Father of Genetics

– was an Austrian monk who was the first to make careful observations across generations of plants.

• his insights led to the birth of the science of genetics

Mendel Video

GENETICS• Genetics: study of how inheritable characteristics are passed from

parent to offspring

GENETIC CHARACTERISTICS• characteristic: an observable feature that can appear in

more than one ex. hand clasping.

• trait: a variation that can exist for a characteristic

ex. clasping your hands with your right thumb on top

5/23/2017

2

Hitchhiker’s Thumb

Tongue Roller

• traits are determined by the genes on the chromosomes

• gene: a segment of DNA that determines a trait

• Chromosomes come in homologous pairs, thus genescome in pairs.Homologous pairs –matching genes – one from mother and one from father

• Example: Humans have 46 chromosomes or 23 pairs.One set from dad – 23 in spermOne set from mom – 23 in egg

INHERITANCE

5/23/2017

3

Gene for eye color(blue eyes)

Gene for eye color(brown eyes)

Homologous pair of chromosomes 

• One pair of Homologous Chromosomes:

Alleles – different genes (possibilities) for the same trait –ex: blue eyes or brown eyes

SOME VOCABULARY:• Phenotype – the trait that is expressed (ex. Blue

eyes) (PHYSICAL APPEARANCE)

• Genotype – the allele combination that the organism has (Ex. bb)

• Homozygous = organism with two of the same alleles

– Aka: purebred

• Heterozygous = organism with two different alleles

– Aka: hybrid

MENDEL’S GENE HYPOTHESIS• the presence of different alleles is responsible for the variation in the

appearance of the organism

• An organism always has 2 genes present for each characteristic (one from each parent).

• purebred: if alleles on both genes are the same(homozygous) BB or bb

• hybrid: if the 2 alleles are different(heterozygous) Bb

LAB TIME:

• Determination of Genotypes and Phenotypes in Humans Activity

5/23/2017

4

MENDEL’S LAWS OF GENETICS• Law of Segregation

– When forming gametes, the parent alleles separate and one copy is passed onto the offspring

• Law of Independent Assortment– During gamete formation, alleles from different genes

have equal probability of combining with each other

• Law of Dominance– Some alleles are dominant and others are recessive.

When one dominant allele and one recessive allele are present (heterozygous), the dominant phenotype will be expressed

PUNNETT SQUARES:• Used to identify probabilities of possible offspring

between organisms

• List possible alleles from one parent on the top and the other parent on the side

• Fill in the chart to find out the possible combinations for the offspring

B

BB = brown eyes (dominant)b = blue eyes (recessive) b

b

• What are the genotypes of the parents?– Are they heterozygous or homozygous? Purebred or

hybrids?

• What are the phenotypes of the parents?

• What are the possible genotypes of the offspring?

• What percentage of the offspring should have brown eyes? Blue eyes?

B

BB = brown eyes (dominant)b = blue eyes (recessive)

b

B PUNNETT SQUARES EXAMPLES

• Outline in your booklet

5/23/2017

5

TYPES OF INHERITANCE:• Not all traits have one dominant and one

recessive allele. We will discuss 4 patterns of inheritance

– Complete dominance (discussed already)

– Incomplete dominance

– Co-dominance

– Sex-linked

INCOMPLETE DOMINANCE• heterozygous organisms express an intermediate

phenotype that is between the two allele options– Ex. RR = red flowers, rr = white flowers, Rr = pink

flowers

• Ex. What is the probability of producing pink flowered pea plants when two heterozygous pea plants are crossed?

CO-DOMINANCE• heterozygous organisms completely express both

of the alleles so they have a new phenotype

– Ex. Blood Type – A and B types display co-dominance

• What are the possible genotypes of the parents of a child with type A blood?

• The mother of a child has type B blood, and the father has type A blood. Is it possible for them to have a child with type o blood? Show work using a Punnett square.

5/23/2017

6

SEX-LINKED• some genes are carried only on the X or the Y

chromosomes, so they are passed differently to boys and girls

– Use XR, Xr or Y to represent a gene carried on an X chromosome

Note: the sex of the baby depends on the chromosome given by the father

• Example: Colour blindness is carried on the X-chromosome. If the mother is colourblind, but the father is not, what is the probability of their son being colour blind? Their daughter?

WHAT IF THERE ARE 2 CHARACTERISTICS WE ARE LOOKING AT?