By the 19th century, most people understood that both parents contributed hereditary material to their children, but the nature of that material was unknown.
Some thought the hereditary material was a fluid that blended during fertilization, but this didn't explain observed traits like freckles or unexpected hair color.
Darwin rejected blending inheritance but couldn't explain heredity, which was central to his theory of natural selection.
Darwin noticed trait variation among individuals; advantageous traits would become more frequent over generations.
Hereditary information (DNA) is divided into discrete units called genes.
An allele is an alternative form of a gene found at the same place on a chromosome, which is critical to understanding heredity.
Mendel
Gregor Johann Mendel (1822-1884) was an Austrian scientist and Augustinian friar.
He carefully bred pea plants and documented the outcomes of his experiments.
Mendel observed that certain traits were passed from plant to plant, generation after generation.
He collected evidence of how inheritance works.
Mendel’s Experimental, Quantitative Approach
Mendel studied the common garden pea, Pisum sativum, which naturally self-fertilizes.
Mendel bred particular individuals together and observed and documented the traits of the offspring.
He controlled reproduction by preventing self-fertilization (removing anthers) and cross-fertilizing by brushing carpels with pollen from another plant.
Mendel collected the seed from these crosses, planted them, and recorded the traits of the new plants.
Many experiments involved plants that "bred true" for a particular trait, meaning all offspring had the same form of the trait as the parent.
Breeders cross-fertilize plants by transferring pollen among individuals with different traits.
Mendel discovered that the traits of offspring often appear in predictable patterns, indicating that hereditary information is passed in discrete units.
Inheritance in Modern Times
Organisms that breed true for a particular trait probably have identical alleles governing that trait.
An individual with identical alleles of a gene is said to be homozygous for that allele.
The particular set of alleles that an individual carries is called its genotype.
Example: Alleles for eye color: B = brown eyes, b = blue eyes; BB, Bb, bb = genotypes
Alleles are a major source of variation in a trait, with new alleles produced by mutation.
A mutation may cause a trait to change, such as a gene for purple flower color mutating to produce white flowers.
Phenotype is an individual’s observable traits; flower color is an example.
Any mutated gene is an allele, whether or not it affects phenotype.
Offspring produced by crossing two individuals that breed true for different forms of a trait are called hybrids.
Hybrids carry different alleles of a gene and are said to be heterozygous.
In many cases, one allele influences the effect of the other, visible in the hybrid phenotype.
Example: Male ligers are sterile, while females are fertile.
Example: Alleles for flower color: B = blue flowers, b = white flowers.
BB = blue flowers (homozygous)
Bb = blue flowers (heterozygous) hybrid
bb = white flowers (homozygous)
An allele is dominant when its effects mask that of the recessive allele paired with it.
Example: Alleles for flower color: B = blue flowers, b = white flowers.
B is the dominant allele.
b is the recessive allele.
Mendel’s Law of Segregation
Mendel crossed pea plants that bred true for purple flowers with pea plants that bred true for white flowers; all offspring had purple flowers.
One gene controls purple flower color in pea plants.
The allele that codes for purple flower color (P) is dominant over the allele that codes for white flower color (p).
Plant with 2 P alleles (PP) has purple flowers.
Plant with 2 p alleles (pp) has white flowers
When homologous chromosomes separate during meiosis, the gene pairs on those chromosomes separate, too.
Each gamete carries only one of the two genes of each pair.
Homozygous dominant (PP) plants can only make gametes carrying the dominant allele.
Homozygous recessive (pp) plants can only make gametes carrying the recessive allele.
If homozygous plants are crossed: PP x pp
Only one outcome is possible: a gamete carrying a P allele will meet up with a gamete carrying a p allele.
All offspring will have one P allele and one p allele.
Genotype = Pp
All offspring will have purple flowers (= phenotype).
A Punnett square is a diagram used to predict the genetic and phenotypic outcome of a cross.
This pattern is predictable and can be used as evidence of a dominant relationship between alleles.
Breeding experiments use these patterns to reveal genotype.
In a testcross, an individual with a dominant trait but unknown genotype is crossed with an individual known to be homozygous recessive for the trait.
The pattern of traits among the offspring can show whether the tested individual is heterozygous or homozygous.
Testcross: Purple flower pea plant x white flower pea plant
Purple flower = PP or Pp (unknown genotype)
White flower = pp (known genotype)
If all offspring had purple flowers, the purple-flowered parent's genotype was PP.
The genotype of the offspring would be Pp.
A monohybrid cross checks for the dominance relationship between the alleles of a single gene.
Individuals identically heterozygous for one gene (Pp) are crossed.
The frequency at which the two traits appear among the offspring may show that one of the alleles is dominant over the other.
Genotypes:
1 PP
2 Pp
1 pp
3 out of the 4 possible outcomes will include at least one copy of the dominant allele, P, and have purple flowers.
Each time fertilization occurs, there is a 1 in 4 chance that the offspring inherits 2 recessive p alleles and has white flowers.
The probability that a particular offspring of this cross will have purple or white flowers is 3 purple to 1 white, represented as the ratio 3:1.
The 3:1 pattern indicates that purple and white flower color are specified by alleles with a clear dominant-recessive relationship.
Purple is dominant.
White is recessive.
Phenotypic ratios in the F2 offspring of Mendel’s monohybrid crosses were close to 3:1.
This formed the basis for his Law of Segregation, which states: “each individual that is a diploid has a pair of alleles (copy) for a particular trait and each parent passes an allele at random to their offspring resulting in a diploid organism”
Monohybrid Cross Example
P = purple flower color
p = white flower color
Two true-breeding flowers crossed: PP x pp
Determine gametes: P, P, p, p
Set up Punnett Square
Do the cross
What is the genotypic ratio of the F1 offspring? 4:0 Pp
What is the phenotypic ratio of the F1 offspring? 4:0 purple flowers.
Monohybrid Cross with F2 Generation
P = purple flower color
p = white flower color
Two F1 offspring crossed: Pp x Pp
Determine gametes: P, p, P, p
Set up Punnett Square
What is the genotypic ratio of the F2 offspring? 1:2:1
1 PP
2 Pp
1 pp
What is the phenotypic ratio of the F2 offspring? 3:1
3 purple flowers
1 white flower
Mendel’s Law of Independent Assortment
During meiosis, gene pairs on homologous chromosomes tend to sort into gametes independently of other gene pairs.
Mendel identified his second law of inheritance by following two characters at the same time.
Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters.
A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring together as a unit or independently.
Using a dihybrid cross, Mendel developed the law of independent assortment.
It states that each pair of alleles segregates independently of any other pair of alleles during gamete formation.
This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.
Genes located near each other on the same chromosome tend to be inherited together.
Dihybrid Cross Example
In rabbits, gray fur (G) is dominant to white (g) fur, and black eyes (B) are dominant to red eyes (b).
A male rabbit with the genotype GGbb is crossed with a female with the genotype ggBb.
What is the male’s phenotype? Gray hair, red eyes
What is the female’s phenotype? White hair, black eyes
How many offspring have gray fur and black eyes? 8
How many offspring have gray fur and red eyes? 8
How many offspring have white fur and black eyes? 0
How many offspring have white fur and red eyes? 0
Dihybrid Cross Another Example
A male rabbit with the genotype GgBb is crossed with a female rabbit with a genotype GgBb.
Genotypes:
Male: GB, Gb, gB, gb
Female: GB, Gb, gB, gb
How many offspring have gray fur and black eyes? 9
How many offspring have gray fur and red eyes? 3
How many offspring have white fur and black eyes? 3
How many offspring have white fur and red eyes? 1
Contribution of Crossovers
Some genes are located on the same chromosome.
Genes are far enough apart that crossing over happens frequently.
They often assort into gametes independently, as if they were on separate chromosomes.
Other genes are very close together on a chromosome and do not assort independently.
Crossing over does not happen very often between these genes.
Such genes are said to be linked.
Alleles of linked genes stay together during meiosis more frequently than others.
The effect of the relative distance between the genes.
Genes that are closer together get separated less often by crossovers.
The closer genes are, the more likely gametes will get parental combinations of genes of those alleles.
Genes that are REALLY close to each other are said to be linked.
A linkage group includes all of the genes on a chromosome.
Dominant vs. Recessive Traits
The terms dominant and recessive describe the inheritance patterns of certain traits.
A dominant trait is expressed.
For a recessive allele to produce a recessive phenotype, the individual must have two copies, one from each parent.
An individual with one dominant and one recessive allele for a gene will have the dominant phenotype.
Beyond Simple Dominance
Not all alleles are clearly dominant or recessive.
An allele can be:
Completely dominant
Incompletely dominant
Codominant with its partner on a homologous chromosome
Incomplete Dominance
In incomplete dominance, an allele is not fully dominant over its partner on a homologous chromosome.
Both are expressed.
The combination of alleles produces an intermediate phenotype.
Snapdragon flower color is an example of incomplete dominance.
R = red pigment
r = no color (mutated allele)
RR plants make lots of red pigment = red flowers
rr plants do not make pigment = white flowers
Rr plants make a little bit of pigment = pink flowers
A cross between 2 pink-flowered plants (Rr x Rr) produces red, pink, and white flowered offspring in a 1:2:1 ratio.
Codominance
Codominant alleles are both expressed at the same time in heterozygotes.
Multiple allele systems, such as ABO blood typing, are examples.
An enzyme coded for by the ABO gene modifies a carbohydrate on the surface of human red blood cells.
A and B alleles code slightly different versions of the enzyme, which modify the carbohydrate a little differently.
O allele has a mutation that prevents its enzyme product from becoming active at all.
The two alleles you carry for the ABO gene determine which form of the carbohydrate you will have on your blood cells.
This determines your blood type.
A and B alleles are codominant when paired.
AB genotype = AB blood type
Can receive blood from any of the other types = universal recipients.
O allele is recessive when paired with either A or B.
AA or AO genotype = A blood type
BB or BO genotype = B blood type
OO genotype = O blood type
Can donate blood to any of the other types = universal donors.
Epistasis
Some traits are affected by multiple gene products.
Epistasis occurs when the expression of one gene is modified (e.g., masked, inhibited, or suppressed) by the expression of one or more other genes.
Also called polygenic inheritance.
Human skin color
Coat color in Labs
Coat color in Labrador retrievers is a good example of epistasis.
Coat color genes only come in black or chocolate.
Yellow Labrador retrievers result when recessive epistatic genes called “extension genes” don't allow color pigment to reach the fur.
Pleiotropy
Pleiotropy is a type of genetic expression in which only one gene affects multiple traits.
Mutations of pleiotropic genes often result in complex genetic disorders.
Sickle-cell anemia
Cystic fibrosis: a disorder that damages your lungs, digestive tract, and other organs.