alternative versions of genes found at the same gene locus 

(brown vs. blue eyes)

1. Choosing the right organism

2. Designing and performing the experiment correctly

3. Analyzing the data properly 

• Pea flowers have stamens, the male structures that produce pollen; sperm are gametes and pollen is the vehicle
• Pea flowers have carpels, female structures housing the ovaries, which produce the eggs
• Pea flower petals enclose both male and female flower parts and prevent entry of pollen from another pea plant

self-fertilize

Pollen from the stamen of a plant transfers to the carpel of the same plant, completing fertilization 

Mendel was able to mate two different plants by hand

Female parts (carpels) were dusted with pollen from other selected plants

• Chose traits that had unmistakably different forms
• Used numerical analysis in studying the traits 

• If a plant is homozygous for purple flowers it will always produce offspring with purple flowers
• Plants homozygous for a characteristic are true-breeding

parental generation

(known as P or P₁)

first filial generation 

(F₁)

Mendel crossed a true-breeding purple flower plant with a true-breeding white flower plant (P generation)

The F₁ generation consisted of all purple flowered plants

The F₂ was composed of 3/4 purple flowered plants and 1/4 white flowered plants a ration 3:1

the results showed that the white trait had not disappeared in the F₁ but merely was hidden 

1. Each trait is determined by pairs of genes; each organism has two alleles for each gene, one on each homologous chromosome

- True-breeding white flowered plants have different allele than true breeding purple flowered plants

2. When two different alleles are present in an organism, the dominant allele may mask the recessive allele

- purple trait is dominant to the white trait

3. The pairs of alleles on homologous chromosomes separate or segregate from each other during meiosis which is known as Mendel's law of segregation

4. Chance determines which allele is included in a given gamete - because homologous chromosomes separate at random during meiosis; the distribution of alleles to the gametes is also random

5. True-breeding organisms have 2 copies of the same allele for a given gene and are homozygous for that gene; hybrid organisms have 2 different alleles for a given gene and are heterozygous for that gene

the particular combination of the two alleles carried by an individual 

(AA vs. aa)

the physical expression of the genotype 

(purple or white flowers)

Let A stand for the dominant purple flowered allele

Let a stand for the recessive white flowered allele

1. First, assign letters to the different alleles of the characteristic under consideration (UPPERCASE for dominant, lowercase for recessive)
2. Determine the gametes and their fractional proportions (out of all the gametes) from both parents
3. Write the gametes from each parent, together with their fractional proportions, along each side of a 2x2 grid (Punnett square)
4. Fill in the genotypes of each pair of combined gametes in the grid, including the product of the fractions of each gamete 
5. Add together the fractions of any genotypes of the same kind (1/4 Aa + 1/1 aA = 1/2 As total)
6. From the sums of all the different kinds of offspring genotypes, create a genotypic fraction (1/4 AA, 1/2 Aa, 1/4 aa is in the ratio 1 AA: 2 Aa: 1 aa)
7. Based on dominant and recessive rules, determine the phenotypic fraction (A genotypic ratio of 1 AA: 2 Aa: 1 aa yields 3 purple flowered 1 white flowered)

1. Cross the unknown dominant-phenotype organism (A_) with a homozygous recessive organism (aa)
2. If the dominant-phenotype organism is homozygous dominant (AA), only dominant phenotype offspring will be produced (Aa)
3. If the dominant-phenotype organism is heterozygous (Aa), approximately half the offspring will be of recessive phenotype (aa)

the sex chromosome carried by the sperm

Sperm recieve either the X or the Y chromosome, along with all of the autosomes

Females only have X sex chromosomes, so the unfertilized egg carries an X chromosome 

Only on the X or only on the Y chromosome

Genes carried on one of the sex chromosomes, but not on the autosomes, are sex linked

Few of the genes on the X chromosome have a specific role in female reproduction.

Most of the genes on the X chromosome have no counterpart on the Y chromosome.

Some genes found only on the X chromosome are important to both sexes (color vision)

recessive alleles of either of the genes located on the X chromosome

The normal, dominant alleles of these genes (called C) encode proteins that allow one set of eye cones to be most sensitive to red light and another to be most sensitive to green light

The afflicted person cannot distinguish between red and green

A man can have the genotype CY or cY

Normal color vision results if his X chromosome bears the C allele or be color blind if his X chromosome bears the c allele

caused by a recessive allele on the X chromosome that results in a deficiency in one of the proteins needed for blood clotting

Hemophiliacs often have anemia owing to blood loss and bruise easily

The hemophilia gene in Queen Victoria of England was passed among the royal families of Europe 

Many traits do not follow simple Mendelian rules of inheritance 

• Not all traits are completely controlled by a single gene
• A trait may not be completely dominant to another 
• Examples include: 

1. Incomplete dominance ("blending")
2. Co-dominance (blood types)
3. Polygenic inheritance
4. Environmental effect

when the heterozygous phenotype is intermediate between the two homozygous phenotypes, the pattern of inheritance is called incomplete dominance

A person with two copies of the C1 allele has curly hair

Someone with two copies of the C2 allele has straight hair

Heterozygotes (with the C1C2 genotype) have wavy hair

If two wavy-haired people marry, their children would have any of the three hair types: Curly (C1C1), wavy (C1C2), or straight (C2C2)

So phenotypic ratio equals the genotypic ratio here (1:2:1)

an individual may have at most two different gene alleles

a species may have multiple alleles for a given characteristic (but each individual still carries two alleles for this characteristic) 

There are 3 alleles in this system: A, B, and O

A and B code for enzymes that add different sugar molecules to the ends of glycoproteins, the o allele does not have any glycoproteins present

These 3 alleles make blood types A, B, AB, and O

alleles A and B are dominant to allele O

People with AA or Ao genotypes have blood type A; people with BB or Bo genotypes have blood type B; people with oo genotypes have blood type O

Consequently, the plasma membranes of their red blood cells have both A and B glycoproteins

When heterozygotes express the phenotypes of both of the homozygotes (in this case, both A and B glycoproteins), the pattern of inheritance is called codominance 

People with type A blood make B antibodies; people with type B blood make A antibodies

People with type O blood make both type A and B antibodies; type AB blood groups make no antibodies

The antibodies cause red blood cells that bear foriegn glycoproteins to clump together and rupture

The presence of such antibodies dictates that blood type must be determined and matched carefully before a blood transfusion is made

Type O blood, lacking any sugars, is not attacked by antibodies in A, B, or AB blood, so it can be transfused safely to all

Type O blood is called universal donor

Because people with type O blood produce both A and B antibodies, they can receive blood only from other type O donors

Their antibodies would attack any donating blood cells bearing A or B glycoproteins 

some characteristics show a range of continuous phenotypes instead of discrete, defined phenotypes

examples of this include human height, skin color, and body build, and in wheat, grain color

The SRY gene on the Y chromosome in male humans encodes a protein that activates other genes

The SRY gene stimulates develoment of gonads into testes and development of the prostate, seminal vesicles, penis, and scrotum 

A siamese cat has the genotype for dark fur all over its bdy

The enzyme that produces the dark pigment is inactive at temperatures about 93 degrees F

In their mother's uterus, the enzyme is inactive and they are born with pale fur everywhere

After birth, the ears, nose, paws, and tail become cooler than the rest of the body, and the dark pigment is produced there 

Eggs hatched between 90 - 93 degrees F become males, and those hatched between 82 and 86 degrees F become females. 

Intermediate temperature ranges produces a mix of both male and females 

often combined with molecular genetics technology to elucidate gene action and expression 

As a result, scientists now know the genes responsible for sickle cell anemia, hemophilia, muscular dystrophy, Marfan syndrome, and cystic fibrosis 

Heterozygous individuals are carriers of a recessive genetic trait (but phenotypically normal)

Recessive genes are more likely to occur in a homozygos combination (expressing the defective phenotype) when related individuals have children

Close relatives are more likely than the general population to each be a heterozygous for a particularl recessive allele and so are more likely to produce the homozygous recessive phenotype

Melanin is the dark pigment that colors skin cells and is produced by the enzyme tyrpsinase. 

An allele known as TYR (for tyrosinase) encodes a defective tyrosinase protein in skin cells, producing no melanin and a condition called albinism 

Humans and other mammals who are homozygous for TYR have no color in their skin, fur, or eyes (the skin and hair appear white and the eyes are pink)

Hemoglobin is an oxygen transporting protein found in red blood cells

A mutant hemoglobin gene causes hemoglobin molecules in blood cells

red blood cells take on a sickle (crescnet) shape and easily break

blood clots can form, leading to oxygen starvation of downstream tissues and paralysis

the condition is known as sickle-cell anemia

People homozygous for the sickle-cell allele synthesize only defective hemoglobin and therefore produce mostly sickled cells

Although heterozygotes have about half normal and half abnormal hemoglobin, they usually have few sickled cells and are not seriously affected 

Because only people who are homozygous for the sickle-cell allele usually show symptoms sickle cell anemia is considered a recessive disorder

a dominant disease can be transmitted to offspring if at least one parent suffers from the disease and lives long enough to reproduce

dominant disease alleles also arise as new mutations in the DNA of eggs or sperm of unaffected parents 

a dominant disorder that causes a slow progressive deterioration of parts of the brain

results in a loss of coordination, flailing movements, personality disturbances, and eventual death

The disease manifests itself in adulthood, ensuring its maintenance in the population 

the incorrect separation of chromosomes or chromatids in meiosis 

It causes gametes to have too many and too few chromosomes

Most embroys that arise from fusion of gametes with abnormal chromosome numbers spontaneously abort but some survive to birth and beyond 

Nondisjunction of sex chromosomes in males produces sperm with either no sex chromosomes (called "0" sperm) or two sex chromosomes (sperm may be XX, YY, or XY)

Nondisjunction of sex chromosomes in females can produce eggs that are O or XX eggs instead of eggs with one X chromosome