Heredity Genetics Chapter 8 CP Biology Heredity Heredity

Heredity & Genetics Chapter 8 CP Biology

Heredity • Heredity, or inheritance, is the passing down of something from one generation to the next • We inherit our traits from our parents • Traits include things like: • • Hair color Eye color Height Freckles Dimples Widow’s peak Hitchhiker’s thumb The list goes on…

Genetics • Genetics is the science of heredity. • Recall that our chromosomes come in pairs called homologues: • Each homologous pair contains one maternal chromosome (inherited from mom) and one paternal chromosome (inherited from dad). • Homologous chromosomes contain the same genes, but may have different versions of some genes. • Different versions of the same

Genetics • For example, you may inherit the allele for freckles from your mother, but the allele for no freckles from your father. • So what determines which trait you actually end up with? • Sometimes one allele is completely dominant over another. • Freckles are a dominant trait, so even with just one allele for freckles, you will have freckles. • Other alleles share dominance, so the trait you see could be a blend of the two. • Many traits (such as eye color) are determined by more than one gene.

Origins of Genetics • Gregor Mendel is considered to be the father of modern genetics. • He was a monk who studied heredity in pea plants. • Pea plants are ideal for studying heredity because: • They reproduce quickly • They have many offspring • They have many easily observable characteristics: • Plant height, flower color, seed color and shape, pod color, etc.

Gregor Mendel studied 7 characteristics in pea plants:

Mendel’s Experiments • Pea plants normally self-pollinate • The pollen from one flower fertilizes the seeds of the same flower • After many generations, the offspring still have the same traits as the parent plants • These are called pure plants – they are homozygous (can be dominant or recessive) • Mendel cross-pollinated pure pea plants with opposite characteristics • Used pollen from purple flowers to pollinate plants with white flowers, and vice versa • In both cases, all of the offspring had purple flowers • When these purple offspring plants were allowed to selfpollinate, they produced both purple-flowered and white-

Parental & Filial Generations • The first plants that are crossed are the parental (P) generation – these plants are pure (truebreeding) plants • The offspring of a parental cross are the first filial (F 1) generation – these plants are called hybrids and all exhibit the dominant phenotype • The offspring of the F 1 plants are the F 2 generation – there are 3 dominant phenotypes for every 1 recessive phenotype

Punnett Squares • Punnett squares are the tool we use to predict the genotypes and phenotypes in the offspring of a given set of • parents. The genotype of one f F parent goes across the top • The genotype of the other parent goes down the side • The parent genotypes are crossed to determine the possible genotypes of the offspring f Ff ff

Punnett Squares and Probability • When 2 heterozygous organisms are crossed, we expect: • A genotype ratio of 1: 2: 1 • A phenotype ratio of 3: 1 • When working with Punnett squares, we are dealing with probability. • The actual results may not exactly match what the Punnett square predicts • The larger the number of offspring, the more closely the actual results match what is predicted.

Law of Segregation • Mendel’s Law of Segregation states that during gamete formation (meiosis), allele pairs separate (segregate), and randomly unite during fertilization. • To test this hypothesis, he crossed a heterozygous plant with a homozygous recessive plant. • He predicted that there would be a 50: 50 ratio of purple to white plants.

This principle can also be used to figure out if an organism is homozygous or heterozygous for a dominant trait

Independent Assortment • The principle of independent assortment: • Traits are inherited independently of one another • This can be shown by a two-trait cross • When both parents are heterozygous for both traits, it is called a dihybrid cross. • In this example, both parents are Rr. Yy: • • R = round seeds r = wrinkled seeds Y = yellow seeds y = green seeds

Dihybrid Crosses • 9 round, yellow seeds • (both dominant traits) • 3 round, green seeds • (one dominant trait, one recessive trait) • 3 wrinkled, yellow seeds • (one dominant trait, one recessive trait) • 1 wrinkled, green seed • (both recessive traits) The phenotype ratio of a dihybrid cross is always 9: 3: 3: 1

Incomplete Dominance • In incomplete dominance, the heterozygous phenotype is intermediate between the two alleles • Ex: Some flowering plants have an allele for red flowers and an allele for white flowers. Plants that have one of each allele will have pink flowers. • Since neither allele is recessive, both are written as capital letters.

Codominance • In codominance, neither allele is dominant, and organisms that are heterozygous will express both phenotypes equally. • Ex: A species of chicken has an allele for black feathers and an allele for white feathers. Chickens that have one of each allele will have both black and white feathers (NOT gray feathers, as you would expect in incomplete dominance) • As in incomplete dominance, both alleles are written as capital letters.

Incomplete Dominance vs. Codominance

Multiple Alleles • There may be more than 2 possible alleles for a trait • Ex: Human blood type • 3 possible alleles: IA, IB, and i • i is recessive • IA and IB are codominant Genotype Phenotype (Blood Type) IAIA or IAi A IBIB or IBi B IAIB AB ii O

ABO Blood Types

The Chromosome Theory of Heredity • In the early 1900 s, Walter Sutton proposed the chromosome theory of heredity: • Genes are carried on chromosomes • This theory was confirmed by Thomas Hunt Morgan, who discovered the X and Y chromosomes while studying fruit flies! • X and Y chromosomes are called sex chromosomes • Females: XX • Males: XY • All other chromosomes are called autosomes (22 pairs in humans)

Sex-Linked Inheritance • The X and Y chromosomes contain other genes besides the ones that determine sex. • The X chromosome is larger and contains more genes than the Y chromosome. • A male needs only one recessive sex-linked allele to have the recessive phenotype

Sex-Linked Inheritance • Sex-linked alleles are written as superscripts on the X chromosome; the Y chromosome gets no superscript. • Ex: Eye color in fruit flies • Red eye allele (dominant): R • • White eye allele (recessive): r Red-eyed females can be XRXR or XRXr White-eyed females must be Xr. Xr Red-eyed males must be XRY White-eyed males must be Xr. Y

Color Blindness is a Sex-Linked Trait

Many Traits Are Determined By More Than One Gene • A polygenic trait is a single trait that is determined by multiple genes • Results in continuous variation – the possibility of many phenotypes from one extreme to another • Modifier genes affect the expression of other genes • Ex: Human eye color – a base color of either brown (B) or blue (b) is determined by one gene, but modifier genes can give people eyes that are green, hazel, gray, etc.