Ch. 11

A Brief History In the past, people did not understand how traits were inherited, but there were many guesses based on things that could be observed. Two theories emerged…. Blending Theory - offspring are a straight mix Particulate Theory traits are inherited as "particles", offspring receive a "piece" from each parent, some pieces may hide the others Golden Doodle

The study of heredity (how traits are passed from one generation to the next)

 Gregor Mendel was the first person to discover how traits are passed from one generation to the next (even though no one knew what DNA or genes were yet).  He did this by experimenting with pea plants.

Mendel’s Peas Mendel did his study on pea plants which have many traits -tall/short -purple /white flowers -round/wrinkled seed

True-Breeding Plants -always create plants that look like themselves Hybrids – offspring of true- breeding plants Tall x Short = Hybrid

Some traits are dominant over others. Tall x Short = all tall offspring (hybrids) *Tall is the dominant trait * Short is recessive

 Molecule that contains the instructions for making all of the proteins needed for life

 DNA wraps around proteins called histones. This forms chromatin.  When the chromatin condenses it forms chromosomes.

 For every chromosome, we inherit two copies.  One from mom and one from dad  Since they are the same type of chromosomes, they are called homologous.

 Genes are passed from parents to their offspring  The adult has two copies of each gene -- one from each parent.  These genes segregated from each other when gametes are formed -The alleles for different genes usually segregate independently of one another.

 Forms of a gene may be dominant while others are recessive

 Each “recipe” on the DNA is called a gene.  Ex. Hair color and hair texture are two different genes found at different loci (locations) on the chromosomes.

 There is often more than one version for a gene.  These are called alleles.  Example: ▪ Gene = flower color Alleles = purple or white

 A dominant allele will ALWAYS be expressed if it is present in the DNA.

 A recessive allele will be hidden or “masked” if a dominant allele is present.  The only way an individual can express a recessive trait is by inheriting two recessive alleles.

 The genotype tells you which alleles an individual has inherited.  Capital letters are used for dominant traits.  Lowercase letters are used for recessive traits.

 The rule is to choose the first letter of the dominant trait.  Example: Brown eyes are dominant over green eyes, so you would use “B” for brown eyes and “b” for green eyes.

 An individual with two different alleles (usually one dominant and one recessive allele). Genotype =

 An individual with two of the same alleles is called homozygous.  Can be homozygous recessive or homozygous dominant.

 The phenotype tells you which trait is physically expressed in the individual.  Example: These puppies’ phenotype is yellow fur.

Explaining the Cross When a parent makes sperm or eggs, their genes separate (PRINCIPLE OF SEGREGATION) The GAMETES (egg or sperm) contain either a T allele (tall) or a t allele (short)