Chapter 9 Patterns of Inheritance

Mendel’s Principles 9.1-9.10

9.1-9.9.10

Gregor Mendel 1822-1884

1. Experimental Genetics by Mendel

Genetics- the scientific study of heredity

Mendel was an Austrian monk who wanted to understand genetics.Mendel studied pea plants to

improve his understanding of how traits are passed from parents to offspring.

Mendel Video

Self-Pollinating-Sperm cells in pollen fertilize the egg cells in the same flowerPea flowers are self-pollinatingThe seeds that are produced by self-

pollination inherit all of their characteristics from the single plant that bore them.

The Experiment:True-Breeding- occurs when plants

produce offspring identical to themselves after self-pollination

Mendel’s pea plants were true-breeding as a result of self-pollination.

Cross-Pollination- when pollen from one flower is delivered to a flower from a different plant.Mendel used cross-pollination to produce

seeds that had two different parents.

He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.

The original parents are represented by P (parental)

The offspring of the original parents are called the F1 generation (1st filial)

The offspring of the F1 generation are called F2 generation (2nd filial)

Hybrid- The offspring of crosses between parents with different traitsEx: the offspring of a cross b/w a red rose &

white rose.The term “crossing” means mating

2. Mendel’s Principle of Segregation

Punnett Square- A tool used to determine the gene combinations that might result from a genetic cross A capital letter is used to represent

the dominant alleleA lower case letter is used to

represent the recessive allele

Allele- alternative forms of genesEx: Gene for flower color; the alleles

are red, purple, and yellow color.

Mendel’s HypothesesIf there are two or more alleles for a gene,

some of the alleles may be dominant and others may be recessive

A sperm cell has only one allele for each trait

An egg cell has only one allele for each trait

Dominant allele- always expressedRecessive allele- only expressed in the

absence of the dominant allele

Homozygous- Organisms that have two identical alleles for a particular trait organisms are true-breeding for a

particular trait. Two alleles for black fur

Heterozygous- Organisms that have two different alleles for the same trait organisms are hybrid for a particular

trait.One allele for black fur and one allele for

white fur

Genotype- genetic make up of the organismEx: genes for black fur and white furEx: Tall (T) allele and short (t) allele

Phenotype- the physical characteristics or what the organisms looks like. Ex: the mouse has black fur. Ex: all plants appear tall

After Mendel’s experiments he formulated the principle of segregation

Principle of Segregation- Pairs of genes separate (segregate) when gametes are formed.

3. Homologous Chromosomes Have Two Alleles

Alleles for a gene are at the same locus (location) on homologous chromosomes

4. Dihybrid Cross

Independent Assortment- Genes that segregate (separately) independently do not influence each other's inheritance.

The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes

Mendel did a dihybrid (two trait) cross to see if the separation of one pair of alleles affects the separation of another pair of alleles.Instead of crossing a yellow seed with a green seed; he observed seed color and seed shape together

Seeds that are round (R) and yellow (Y) are dominant to seeds that are wrinkled (r) & green (y)

True-breeding round & yellow (RRYY)seeds were crossed with true breeding wrinkled and green seeds (rryy)

The result: All individuals in the F1 generation were round & yellow

Dihybrid Cross: F2Heterozygous F1 plants (RrYy) were crossed with each other to determine if the alleles would segregate from each other in the F2 generation.

RrYy × RrYy

The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation

9 round & yellow: 3 round & green: 3 wrinkled & yellow: 1 wrinkled & green

The results of the experiment:some seeds were round and yellowsome seeds were wrinkled and greensome seeds were round and greensome seeds were wrinkled and yellow

Mendel had discovered the principle of independent assortment

Independent Assortment explains the variation observed in plants, animals, and other organisms

5. Probability

Probability can be used to predict the results of genetic crosses.

Probability- the likelihood that something is going to happen.In genetics expressed as a ratio or percentage.

Ex: the probability that it will snow in March is 50:50

Probability of Blue vs. Green Beetle

Probabilities predict the average outcome of a large number of events.

Probability cannot predict the exact outcome of an individual event

In genetics, the larger the number of offspring, the closer the resulting numbers will get to the expected values.

6. Human Traits Tracked W/ Family Pedigrees

Geneticis who study humans triats study matings that have already occurred.

Pedigree charts- shows relationships w/i a family of traits controlled by a single gene and not by environmental influences.

Ex: ability to taste PTC (a bitter taste)ColorblindnessAlbinism (lack of pigment in skin, hair, & eyes)

People who are heterozygous for a trait are called carriers

Ex: DD (normal hearing)Dd (carrier/heterozygous)dd (deaf)

7. Single Gene Inherited Disorders in Humans

There are two types of inherited disorders in humans; recessive & dominant

Recessive DisordersMost human disorders are recessiveCystic Fibrosis is common in the U.S. Excessive mucus secretion clogs the

lungsFew black people have the disease (1/17,000)More white people have the disease (1/1,800)

Dominant DisordersOnly copy of allele needs to present for the

disordered to have the disease

8. Fetal Testing Can Detect Disorders during Preganancy

Most genetic disorders can be detected before the birth

Test done between weeks 14 and 20 of the pregnancy can determine genetic disordersAmniocentesis- taking a sample of the

amniotic fluid and testing itChorionic villus sampling (CVS)-small

amount of fetal tissue is suctioned from the placenta; karyotyping is performed & chemical analysis

Ultrasound Imaging- an image of the fetus is used to determine anatomical deformities