Chapter 14 Mendel and Gene Idea

Particulate Hypothesis

• Idea that parents pass on discrete heritable units or genes that retain their separate identities in offspring

• Genes can be passed on, generation after generation, in undiluted form

• Comparison to deck of cards or a bucket of marbles

Mendel’s Approach• Character- a heritable feature such as flower color• Trait – choice of color such as purple or white• Mendel’s pea plants were true breeding plants which

meant that if the parent had purple flowers the offspring would be the same

• Hybridization – Crossing of two true breeding plants parents – P1 generationoffspring – F1 generationF1 offspring – F2 generation

• Analysis of F2 generation that gave rise to fundamental principles of heredity.

Law of SegregationFour principles of Law of Segregation:

1. Alternative versions of genes account for variations in inherited characters. Now known as alleles.2. For each character, an organism inherits two alleles, one from each parent.3. If the two alleles at a locus differ, the dominant allele determines the appearance, the recessive has no noticeable effect.4. Two alleles for a heritable character separate during gamete formation and end up in different gametes. (corresponds to homologous chromosomes distributed to different gametes in meiosis)

Alleles in DNA

Terminology

• Homozygous alleles – 2 of the same for a character

• Heterozygous alleles – 2 different alleles for the same character

• Phenotype – physical traits or outward characteristics

• Genotype – genetic makeup or the alleles themselves

Law of Independent Assortment

• States that each pair of alleles segregates independently of other pairs of alleles during gamete formation

*monohybrid cross – tests one character

*dihybrid cross – tests 2 characters and gives rise to 4 possible phenotypes

based on how genes separate into each gamete

Dihybrid Cross

Laws of Probability

• Coin tosses, rolling dice, or drawing cards from a deck• Probability that an event will happen is 1• Multiplication rule: states that to determine the probability of an

event with 2 variables we must multiply probability of one event by the probability of another event

ex: 2 coins landing on heads½ X ½ = ¼

• Addition rule: Probability of any one of 2 or more mutually exclusive events occurring is calculated by adding together their individual probabilities

ex: 2 coins each heads ¼ + ¼ = ½

Inheritable Patterns• Spectrum of dominance – different

degrees of dominance and recessiveness in relation to each other.

*Complete dominance – one allele is displayed over the other

*Incomplete dominance – creates a third trait somewhere between true breeds

Ex: red X white = pink*Codominance – two alleles affect the phenotype in

separate waysEx: blood typing – A, B,

AB, O

Relationship between Dominance and Phenotype

• Dominance isn’t established from one trait subduing the other

• Dominance can be an allele coding for an enzyme that allows proper functioning processes to occur, whereas the recessive allele codes for the defective form of the enzyme and can lead to inherited disorders

Ex: Tay-Sachs disease – inability to metabolize certain lipids leads to seizures, blindness, and motor or mental degeneration

Multiple Alleles

• ABO blood system in humans has 4 possible phenotypes

Allele Effects

• Pleiotropy – a single gene can phenotypically affect multiple characteristics in an organism

Ex: cystic fibrosis & sickle cell • Epistasis – a gene at one locus alters the

phenotypic expression of a gene at a second locus.Ex: black vs. brown coat color in mice-epistatic gene is pigment deposited

in the coat; recessive locus could produce white coat

Epistasis Example

Polygenic Inheritance

• Polygenic inheritance - Indicates that multiple genes could affect a single phenotypic character

- Ex: skin pigment in humans- opposite of pleiotropy

Pedigree Analysis

• Pedigree – collects information about a family’s history for a particular trait and assembles it into a tree describing interrelationships of parents and children across generations

*can be used to predict future occurrences of a character

Pedigrees

Recessive Inherited Disorders

• Individuals that inherit recessive alleles from both parents are targets

• Typically recessive allele codes for a malfunctional protein or no protein at all that leads to disorder

• Heterozygotes are labeled as carriers, even though they function normally

Recessive Inherited Disorders

• Cystic Fibrosis – allele codes for defective chloride transport channels or no channels which leads to build up of thick, sticky mucus in the pancreas, lungs, and digestive tract.

-Ultimately leading to increase chronic bronchitis, bacterial infections, and poor absorption of nutrients

• Sickle-Cell disease – substitution of a single amino acid in hemoglobin of red blood cells leads to clumping or clogging of blood vessels from deformed hemoglobin molecule

Dominantly Inherited Disorders

• Achondroplasia – form of dwarfism*affects 1 in 25,000*recessive allele is more prevalent than

dominant allele• Huntington’s disease – degenerative disease of the

nervous system*affects 1 in 10,000*high incidence lead to discovery of locus in

tip of chromosome 4*a test has been developed to detect presence of

Huntington’s allele

Identifying Carriers• Fetal Testing:

1. Amniocentesis – between 14th and 16th week of pregnancy can determine presence of disorder by examining extracts from amniotic fluid in uterus

2. Chorionic villus sampling – during 8th to 10th week of pregnancy, uses a small suctioned sample of placenta to detect presence of disorder

• New born screening:phenylketonuria – impaired ability to break down

phenylalanine which can lead to mental retardation-1 in 10,000 to 15,000 affected-detection can lead to altered diet to promote normal

development