Chapter 1

genes

the basic units of biological information

heredity

the way genes transmit traits from parents to offspring

what underlie the formation of every heritable trait

genes

What is genetics?

the science of heredity

Who discovered the basic principles of genetics in the mid-nineteenth century

Gregor Mendel - Augustinian monk

• published his findings in 1866 (7 yrs after Darwin’s book appeared in print)

What are the 4 general themes that emerge from Mendel’s work?

• Genetic variation is common – Differences in traits are widespread and provide the diversity needed for evolution.

• Variation helps track inheritance – Trait differences make it possible to follow how genes are passed from one generation to the next.

• Inheritance follows predictable patterns – Mendel’s laws explain how traits are inherited, accounting for both similarities and differences among family members.

• Mendel’s laws are universal – The principles of heredity apply to all sexually reproducing organisms, from plants to humans.

What were the 2 major misconceptions prior to Mendel?

1. One parent contributes the most to an offspring’s inherited feature (male), by Nicolaas Hartsoeker, by the way of the fully formed homunculus inside the sperm

2. Blended inheritance — parental traits become mixed and forever changed in the offspring

What did Mendel do differently?

1. Chose the garden pea as experimental organism (can both self and cross fertilize, and can obtain large numbers in short periods)

2. Examined clear-cut alternative states of particular traits (no intermediate traits existed)

3. Collected and perpetuated lines of peas that bred true

4. Made reciprocal crosses — which demonstrated that two parents contribute equally to inheritance

5. Worked with large numbers of plants and performed numerical analysis

What are pure-breeding/true-breeding lines

produce offsprings that remain constant from generation to generation

also called “inbred” — have been mated only to each other for many generations

What are antagonistic pairs

constant but mutually exclusive alternative (ex. purple vs white flowers), which produced hybrids

How did Mendel explain the 3:1 ratio

He proposed that…

1. For each trait we carry 2 copies of a unit of inheritance (genes), one from each parent

2. Each gene comes in alternative forms (alleles), and a combination of the alternative forms determine the contrasting characteristics

what are monohybrids

individuals having 2 different alleles of a single gene

What is Mendel’s Law of segregation?

“The two alleles of each gene separate during gamete formation, and then unite at random, one from each parent, at fertilization”

meaning — each gamete has only one copy of the allele, and during fertilization the two alleles will unite at random

What is the product rule?

the probability of 2 or more independent events occurring together is the product of the probability that each event will occur by itself

Probability of event 1 x Probability of event 2

What is the sum rule?

The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities

Probability of event 1 + Probability of event 2

Homozygote vs Homozygous vs Heterozygote vs heterozygous

Homozygous → A YY or yy genotype

Homozygote → an individual with a homozygous genotype

heterozygous → A Yy genotype

Heterozygote (aka hybrid) → an individual with a heterozygous genotype

What is Mendel’s law of independent assortment?

“During gamete formation, different pairs of alleles segregate independently of each other”

What are the different parts of a pedigree

What is a late onset genetic trait

symptoms are not present at birth and manifest themselves only later in life

What happens in the Huntington disease?

This illness usually shows up in middle age and slowly destroys its victims both mentally and physically. Symptoms include intellectual deterioration, severe depression, and jerky, irregular movements, all caused by the progressive death of nerve cells. If one parent develops the symptoms, his or her children have a 50% probability of suffering from the disease, provided they live sufficiently long.

What does “vertical” pattern of transmission mean?

If you trace back through the ancestors of any affected individual, you would see at least one affected person in each generation, giving a continuous line of family members with the disease

vertical pattern is a strong evidence that a dominant allele causes the condition

What is horizontal transmission?

when the parents, grandparents, and great-granparents of the children that are born with the disease do not themselves have the disease

Strong indication of the trait being recessive

How to recognize dominant and recessive traits in pedigrees?

Dominant

• affected children always have at least one affected parent

• dominant traits show a vertical pattern of inheritance: the trait shows up in every generation

• two affected parents can produce unaffected children, if both parents are heterozygotes

Recessive

• affected individuals can be the children of two unaffected carriers, particularly as a result of consanguineous marriage (usually second cousins or closer — indicated by the double line)

• all the children of two affected parents should be affected

• race recessive traits show horizontal pattern of inheritance: the trait first appears in one or more members of one generation and is not seen in earlier generations

• recessive traits may show a vertical pattern of inheritance if teh trait is extremely common in the population