genetics chapter 12. 12-1: origins of hereditary science

Genetics

Chapter 12

12-1: Origins of Hereditary Science

Mendel’s Experiments

Genetics – The study of how characteristics are transmitted from parents to offspring

Heredity – The passing on of genetic characteristics from one generation to another

Mendel’s Experiments

Gregor Mendel (an Austrian monk) used cross-pollination techniques in which pollen is transferred between flowers of two different plants.

Mendel’s Experiments

Reasons Mendel used pea plants Require little care Produce many offspring Mature quickly (8-10 weeks)

Mendel’s Experiments

P (parental) generation – Organisms that are true-bred for specific traits

F1 generation – Offspring of the P generation

F2 generation – Offspring of the F1 generation F = Filial (pertainting to a son or daughter)

Mendel’s Experiments

Self-pollination – A plant with both male and female reproductive organs that fertilizes itself

Cross-pollination – Pollen from one plant is carried to the flower of another plant

Mendel’s Experiments

Three Steps of Mendel’s Experiments

Mendel’s Experiments

Gene – A section of DNA that carries the information on how to make one protein

Mendel’s Experiments

Allele – An alternative form of a gene

One allele for each trait is passed from each parent to the offspring.

Mendel’s Experiments

Dominant trait – An allele that is fully expressed whenever the allele is present in an individual The trait that masks the

other Recessive trait – An allele

that is expressed only when there is no dominant allele present in an individual The trait that is masked

12-2: Genetic Crosses

Monohybrid Cross

Monohybrid cross – A cross in which one characteristic is tracked

Punnett square – used to predict the outcome of genetic crosses

Monohybrid Cross

Genotype – the genetic makeup of an organism (use letters)

Phenotype – the appearance of an organism (physical appearance)

Monohybrid Cross

Homozygous – having identical alleles Heterozygous – having different alleles

Monohybrid Cross

Trait – Plant height Phenotype

T – tall t – short

Genotype TT – homozygous

dominant (pure bred) Tt – heterozygous

(hybrid) tt – homozygous

recessive (pure bred)

Monohybrid Cross

Probability – the likelihood that a specific event will occur

Probability = number of one kind of possible outcome

total number of all possible outcomes

A probability may be expressed as a decimal, a percentage, or a fraction

Mendel’s Characteristics

Monohybrid Cross

Ratios and percentages TT x tt = 100% heterozygous TT x TT = 100% homozygous dominant tt x tt = 100% homozygous recessive Tt x tt

50% heterozygous 50% homozygous recessive

Monohybrid Cross

Ratios and percentages Tt x Tt = Genotypic ratio – 1:2:1 ratio

25% homozygous dominant 50% heterozygous 25% homozygous recessive

Tt x Tt = Phenotypic ratio – 3:1 75% tall 25% short

Dihybrid Cross

Dihybrid cross – A breeding experiment that tracks the inheritance of two traits

Example Traits: Seed shape & Seed color Alleles: R – round

r – wrinkled Y – yellow y – green

Dihybrid Cross

Use the FOIL method for each parent to set-up the Punnett square

RrYy x RrYy

RY Ry rY ry RY Ry rY ry

Dihybrid Cross

RY

Ry

rY

ry

RY Ry rY ry

24

RRYY

RRYy

RrYY

RrYy

RRYy

RRyy

RrYy

Rryy

RrYY

RrYy

rrYY

rrYy

RrYy

Rryy

rrYy

rryy

Round/Yellow: 9

Round/green: 3

wrinkled/Yellow: 3

wrinkled/green: 1

9:3:3:1 phenotypic ratio

RY Ry rY ry

RY

Ry

rY

ry

Dihybrid Cross

This chart is used to illustrate the variety of eye colors

The trait for eye color actually has three alleles:

AaBbCc

Why eye color changes in people is unknown

Other Monohybrid Crosses

Incomplete dominance – a form of intermediate inheritance in which one allele for a specific trait is not completely dominant over the other allele This results in a

combined phenotype.

Other Monohybrid Crosses

Codominance – Two alleles are expressed (multiple alleles) in heterozygous individuals Two alleles that are both dominant

Example: blood typesKey

A = IA

B = IB

O = I

A and B are dominant over O

Other Monohybrid Crosses

Genotypes Phenotypes Type A = IAIA or IAi AA or AO Type B = IBIB or IBi BB or BO Type AB = IAIB AB Type O = ii OO

Other Monohybrid Crosses

Sex-Linked traits – Traits (genes) located on sex chromosomes X – Female sex chromosome Y – Male sex chromosome

Many sex-linked traits carried on X chromosome

Pedigree

Pedigree – A diagram of family relationships that uses symbols to represent people and lines to represent genetic relationships