HEREDITY

Chapter 5

• Heredity is the passing of traits from parent to offspring

• Genetics is the science that studies how traits are inherited

• Alleles are the different forms of a trait that a gene may have. – alleles determine traits–A gene is a section of DNA on a chromosome

• Gregor Mendel is known as the father of genetics

– in 1856, Mendel experimented with pea plants using the scientific method

–he traced one trait of the plants through several generations

• An organism that always produces the same traits generation after generation is called a purebred.

• A hybrid receives different information (different alleles) for a trait from each parent

B

Purebred Labrador Retrievers Purebred Standard Poodles

<- < Hybrid Labradoodles

DOMINANT AND RECESSIVE ALLELES:

• A dominant trait covers over (or dominates) another form of the trait

• Recessive describes a trait that is covered over, or dominated, by another form of the trait- a recessive trait seems to disappear in an

organism

• A Punnett Square is a tool used to predict results in genetics.- In a Punnett Square, letters represent dominant

and recessive alleles- Capital letters represent dominant alleles;

lowercase letters represent recessive alleles

• The letters in a Punnet Square are a form of code- they show the genotype, or genetic makeup of

an organism

This Punnet Square shows the genotype for the height of an organism

This Punnett Square shows the genotype for eye color

• A phenotype is the outward appearance and behavior of an organism as a result of its genotype.- for example, if you have blue eyes, the

phenotype for your eye color is blue

https://www.23andme.com/gen101/phenotype/

• An organism with two alleles that are the same is homozygous.- on a P.Sq, they are represented with the same

form of the same letter. Ex: TT or tt

• An organism that has two different alleles for a trait is called heterozygous- represented using different forms of the letter

on a P.Sq. EX: Tt

MENDEL’S PRINCIPLES OF HEREDITY:

1. Traits are controlled by alleles on chromosomes

2. An allele’s effect is either dominant or recessive

3. When a pair of chromosomes separates during meiosis, the different alleles for a trait move into separate sex cells

• Incomplete dominance refers to the production of a phenotype that is intermediate (“mid-way”) between two homozygous parents. The dominant trait is then incomplete.

- For example, if crossing a red flower and a white flower produces a pink flower, the red trait is present, but does not completely dominate

• Polygenic inheritance occurs when a group of gene pairs acts together to produce a trait

- In humans, eye color, skin shade, hair color, height, etc… all are results of polygenic inheritance

Punnett Square for possible skin shades

• Environmental factors that effect genes may be internal or external

- Internal influences include chemicals in the body

- External influences include the living conditions and habits of the organism (smoking, diet, etc)

• A mutation is a permanent change in a gene or chromosome

• Mutations are a result of errors in copying DNA

• One common chromosome disorder in humans is Down Syndrome

• Down syndrome results when three copies of chromosome #21 are made, instead of two copies

• A recessive genetic disorder is caused by a recessive gene

- This means that the parent who carries the gene for the disorder does not have the disorder, but can pass it on to their offspring

- Cystic fibrosis is a common recessive genetic disorder

• Some inherited conditions are linked with the X and Y chromosomes, which determine gender of offspring

• These are called sex-linked disorders, because they are caused by sex-linked genes.

• Color blindness is a sex-linked disorder

• A tool used for tracing a trait through the generations of a family is called a pedigree

• Pedigree is used to determine patterns of inheritance

• This helps geneticists to predict the probability that a baby may be born with a specific trait–Diseases and disorders; traits in animal

breeding