Study of heredity

• Heredity: passing of traits from parent to child

• Patterns of relatedness can help predict offspring characteristics

• Genes are located on chromosomes• Genes inherited from mom and dad• Genes come in several forms called alleles

– Allele: Alternative forms of a gene

– Ex: Flower color• White allele or Purple

allele

• An allele is any alternative form of a gene occurring at a specific locus on a chromosome.

– Each parent donates one allele for every gene.

– Homozygous describes two alleles that are the same at a specific locus.

– Heterozygous describes two alleles that are different at a specific locus.

Alleles can be represented using letters. Ex: Aa or AA or aa

– A dominant allele is expressed as a phenotype when at least one allele is dominant.

– A recessive allele is expressed as a phenotype only when two copies are present.

– Dominant alleles are represented by UPPERCASE letters

– Recessive alleles by lowercase letters.

Dominant and Recessive Genes•Gene that prevents the other gene from “showing” – dominant

•Gene that does NOT “show” even though it is present – recessive

•Symbol – Dominant gene – upper case letter – T Recessive gene – lower case letter – t

Dominant color

Recessive color

•Combination of genes an organism has (actual gene makeup) – GENOTYPE Ex: TT, Tt, tt

•Physical appearance resulting from gene make-up – PHENOTYPE

Ex: hitchhiker’s thumb or straight thumb

Genotype and Phenotype

Genetics Vocabulary ReviewChoices:

1) YY

2) Tall

3) Yy

4) SS

5) yY

6) gg

7) Smooth

8) Green

9) TT

10)Dwarf

Which choice(s) are examples of:

Genotypes?

1, 3, 4, 5, 6, 9

Phenotypes?

2, 7, 8, 10

Homozygous genotypes?

1, 4, 6, 9

Heterozygous genotypes?

3, 5

Homozygous recessive genotypes?

6

Homozygous dominant genotypes?

1, 4, 9

Dominant vs. Recessive

• If Brown eye color is dominant over blue eye color, how would you indicate…– The brown allele?

– The blue allele?

#1

Dominant vs. Recessive

• If dimples in the cheeks are dominant, how would you indicate…

– The allele for dimples?

– The allele for no dimples?

#2

Dominant vs. Recessive

• If left thumb over right thumb is dominant, how would you indicate…

– Left over right?

– Right over left?

#3

Mendel laid the groundwork for genetics.

• Traits are distinguishing characteristics that are inherited.

• Genetics is the study of biological inheritance patterns and variation.

• Gregor Mendel showed that traits are inherited as discrete units.

• Many in Mendel’s day thought traits were blended.

Mendel’s data revealed patterns of inheritance.

• Mendel made three key decisions in his experiments.– use of purebred plants

– control over breeding

– observation of seven“either-or” traits

• Mendel used pollen to fertilize selected pea plants.

Mendel controlled thefertilization of his pea plantsby removing the male parts,or stamens.

He then fertilized the femalepart, or pistil, with pollen froma different pea plant.

– P generation crossed to produce F1 generation

– interrupted the self-pollination process by removing male flower parts

• Mendel allowed the resulting plants to self-pollinate.

– Among the F1 generation, all plants had purple flowers

– F1 plants are all heterozygous

– Among the F2 generation, some plants had purple flowers and some had white

Each trait had a 3:1 ratio.

No way that can be a coincidence!

1. Law of Dominance• If the two alleles of a particular gene present in an

individual are the same, the individual is said to be homozygous. (dominant or recessive)

• If the alleles of a particular gene present in an individual are different, the individual is heterozygous.

• In heterozygous individuals, only the dominant allele is expressed; the recessive allele is present but unexpressed.

Mendel’s 3 Conclusions

2. Traits are inherited as discrete units.

• Organisms inherit two copies of each gene (allele), one from each parent.

• Genes are inherited independently of each other.

purple white

Mendel’s 3 Conclusions

Mendel’s 3 Conclusions

3. Law of Segregation

The two copies of alleles segregate during gamete formation.

• Defined: Tools used to determine genetic probability

• Crosses:

– Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.

– Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.

• Probability = likelihood that a certain event will happen

Punnett Squares

Cross a homozygous dominant yellow (YY) plant with a homozygous recessive green (yy) plant

Key

Y = yellow

y = green

Probability of growing a yellow plant? _____________

Probability of growing a homozygous recessive plant? ____________

100%

0%

Probability of growing a homozygous dominant plant? ____________0%

Cross a heterozygous dominant yellow (Yy) plant with a heterozygous dominant yellow (Yy) plant

Key

Y = Yellow

y = green

Probability of growing a heterozygous plant? ________

Probability of growing a green plant? ________

Probability of growing pure yellow? _________

50%

25%

25%

Maria is a heterozygous healthy (Hh) female and Jeff suffers from recessive cystic fibrosis (hh).

Key

H = healthy

h = cystic fibrosis

Probability of having a healthy child? _________

Probability of having a homozygous dominant child? ________50% 0%

PEDIGREE

• A pedigree chart is a diagram that shows the occurrence particular gene/trait from one generation to the next.