Genetics and Heredity

Inheriting Traits

• Eye color, nose shape and many other physical features are some of the traits that are inherited from parents.

• An organism is a collection of traits, all inherited from it parents.

Vocabulary

Heredity – passing of traits from parent to offspring

Genetics – the study of traits passed from parents to offspring

Trait – genetically determined variant of a characteristic

Trait vs. Characteristic – if a characteristic is “eye color”, blue eyes would be a possible trait

Alleles – different forms of a trait

• Every sex cell has one allele for each trait

• Genetics is the study of how traits are inherited through the interactions of alleles

Father of Genetics• Gregor Mendel began experimenting with

garden peas in 1856• Carefully observed the pea plants, resulting in the

first recorded study of how traits pass from one generation to the next

• Used the math of probability to explain heredity

• The first to trace one trait through several generations

Genetics in a Garden

• Each time Mendel studied a trait, he crossed two plants with different expressions of the trait and found that the new plants all looked like one of the two parents.

Genetics in a Garden

• He called these new plants hybrids because they received different genetic information, or different alleles, for a trait from each parent.

Genetics in a Garden

Purebred – an organism that always produces the same traits generation after generation

Ex. Tall plants that always produce seeds that produce tall plants are purebred for the trait of tall height

Self-pollination – when pollen from a plant is transferred to a flower on the same plant

Cross pollination – when pollen from a plant is transferred to a flower on a different plant

*In his experiments, Mendel used pollen from the flowers of purebred tall plants to pollinate by hand the flowers of purebred short plants

• Mendel found that tall plants crossed with short plants produced all tall plants.

• DOMINANT vs. RECESSIVE

• DOMINANT – Mendel called the tall form dominant because it dominated, or covered up, the short form

• RECESSIVE – He called the form that seemed to disappear the recessive factor

Probability – Make a prediction

• Mendel used probability (a branch of math that helps you predict the chance that something will happen.)

• His predictions were accurate because he worked with a large number of plants (almost 30,000 pea plants in 8 years), thereby increasing his chances of seeing a repeatable pattern.

Punnett Squares• A tool used to

predict results in genetics is the Punnett square. It helps you predict what offspring would look like.

• In a Punnett square, letters represent dominant and recessive alleles.

An uppercase letter stands for a dominant allele

An lowercase letter stands for a recessive allele

• Punnett squares show the genotype or the genetic makeup of an organism inherited from its parents

• It also shows the phenotype, which is the appearance of an organism (ex. Tall or short)

• Most cells in your body have two alleles for every trait. The alleles are located on chromosomes within the nucleus.

Ex. Trait - HeightT allele would be for

Tallt allele would be for

short

• An organism with two alleles that are the same is called homozygous. Ex. TT

• An organism that has two different alleles for a trait is called heterozygous. Example Tt

Making a Punnett Square

B B

b

b Bb Bb

Bb Bb

Dominance

• An allele’s effect is Dominant or recessive. • More common traits tend to be dominant

and less common are recessive.Ex. T – Tall, t – short

TT would be TallTt would still be Tall (because big T is

dominanttt would be short

Activity

1. What fraction of this couple’s children would you expect to be AA?2. What fraction of this couple’s children would you expect to be Aa?3. What fraction of this couple’s children would you expect to be aa?4. What fraction of this couple’s children would you expect to have

normal pigmentation?5. What fraction of this couple’s children would you expect to have

albinism?

A = normal pigmentation

a = albinism

Mendel’s Laws of Genetics

1. Law of Segregation2. Law of Independent Assortment

1. Law of Segregation

• For any particular trait, the pair of alleles of each parent separate and only one allele passes from each parent on to an offspring.

• Which allele in a parent’s pair is inherited is a matter of chance.

Ex. Each parent gives only one allele to an egg or sperm. When fertilization occurs, the offspring’s gene pair is determined by which allele each sex cell carried.

2. Law of Independent Assortment

• Different pairs of alleles are passed to offspring independently of each other.

• This means that the offspring can have combinations of genes that neither parent has. So, the offspring can look differently than both parents.

Ex. Explains why the human inheritance of a particular eye color does not increase or decrease the likelihood of having 6 fingers on each hand.

Types of Genetic Crosses

• Monohybrid cross – cross involving single trait

Ex. Flower color• Dihybrid cross – cross involving two

traitsEx. Flower color and plant height

More Words

• P Generation – parent generation in a genetic cross

• F1 generation – first generation offspring resulting from a cross between parents

• F2 generation – second generation offspring resulting from a cross between the F1 offspring

Sex Determination

XX – girlsXY – boys

Females produce eggs with X chromosomes only.

Males produce sperm with X and Y chromosomes

Sex-Linked Disorders

• An allele inherited on a sex chromosome is called a sex-linked gene.

Ex. Color blindness is a sex-linked disorder in which people cannot distinguish between certain colors, particularly red and green

• This trait is a recessive allele on the X chromosome.

• Because males have only one X chromosome, a male with this allele on his X chromosome is color blind.

• A color blind female occurs only when both of her X chromosomes have the allele for this trait.

So, are you color blind or not?

Pedigree

• A visual tool for following a trait through generations of a family.

• Males – squares, Females – circles

• Completely filled circle or square – trait is seen in that person

• Half colored – indicate carriers

• Empty – do not have the trait and are not carriers

Homework

1. You are newly married and want to find out the probability of you having kids with blue eyes. You have brown eyes, while your spouse has blue eyes. Will you have kids with blue eyes? And if so, how many?

2. What other sex-linked genetic disorders are there? Give examples and a brief description.