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Who is considered to be "the father of modern genetics," and which organism did he famously study?

The "father of modern genetics" is Gregor Mendel, and he studied pea plants.

Mendel, pictured here, was a monk who conducted heredity experiments in the 19th century - before the term "gene" had even been coined. Heredity that follows the patterns that Mendel discovered is still termed "Mendelian genetics" today.



Genetics is the study of heredity and inherited traits.

On the AP Biology exam, you will likely face questions related to both classical genetics (Punnett squares, dominance and recessivity, etc.) and molecular genetics (DNA replication, nucleotides, etc.).



A gene is a nucleic acid sequence that determines some trait of an organism. In eukaryotes, genes are composed of DNA and located on chromosomes.

Many aspects of inheritance were discovered by Gregor Mendel, though the term "gene" had not yet been coined. Instead, Mendel called genes "factors."


What term describes the position of a particular gene on a chromosome?

The chromosomal position of a gene is called a locus (plural: loci).

Each locus falls at the same relative position in a species. This allows the construction of genetic maps.



An allele is a variation of a specific gene, usually denoted by a single letter. Humans always have two alleles at each genetic locus.

For example, the B and b alleles might code for brown and blue eye color, respectively.



A phenotype refers to a physical or observable characteristic of an organism, determined by its genotype.

For example, if a pea plant is homozygous for the Y allele and is yellow in color, "yellow" would be its phenotype.



A genotype refers to the actual set of alleles possessed by an organism.

For example, if a pea plant is homozygous for the Y allele and is yellow in color, "YY" would be its genotype.


What is the term "wild type" used to signify?

Wild type refers to the allele or phenotype that naturally predominates in a population.

For example, experiments involving bacteria often compare a wild-type strain with one or more sets of mutants. Wild-type bacteria exist as they would in nature, while mutants either lack a normal function or gain an abnormal one.


Explain the difference between a homozygous and a heterozygous genotype.

A homozygous genotype includes two copies of the same allele, while a heterozygous individual possesses two different alleles.

Let's say that a plant species has two alleles that determine height, T (tall) and t (short). TT and tt individuals would be homozygous while Tt organisms would be heterozygous.


complete dominance

Complete dominance is the simplest inheritance pattern tested on the AP Biology exam. One allele is dominant, meaning that it determines the phenotype whenever present. The other allele is recessive and only affects the phenotype when the dominant allele is not present.

Mendel observed complete dominance in his experiments with pea plants.


In a certain population, the R allele codes for red color and the r allele codes for white color. If 100% of individuals with an Rr genotype appear red, how would we describe the r allele?

The r allele must be recessive.

In complete dominance, heterozygotes always display the dominant phenotype. In this example, that allele would be R. The recessive allele is completely "masked" in such cases.


For a particular trait, the dominant allele is denoted as A, while the recessive allele is denoted as a. With regard to this trait, what must be the genotype of a heterozygous individual?


Heterozygous individuals have one dominant and one recessive allele. Note that a homozygous dominant individual would have a genotype of AA, while a homozygous recessive individual would have a genotype of aa.


Name Mendel's two laws.

  • the Law of Segregation (1st Law)
  • the Law of Independent Assortment (2nd Law) 

Mendel also proposed a third law, the Law of Dominance, which explained that dominant alleles will always be displayed, while recessive alleles will be "hidden" when a dominant allele is also present. However, the Law of Dominance is currently categorized as a principle, not technically a law.


Explain Mendel’s Law of Segregation.

An organism carries two alleles for each trait, but these alleles "segregate" during the formation of gametes. Thus, a parent organism will only pass one allele per trait to its progeny.

Though Mendel did not know this at the time, segregation of alleles occurs during anaphase of meiosis I.


Explain Mendel’s Law of Independent Assortment.

Mendel hypothesized that the inheritance of one trait will be unaffected by another. In other words, alleles at different loci assort independently.

This law only holds true when genes are not located on the same chromosome.


Label the diagram below with the P, F1, and F2 generations.

P refers to the parent generation, shown in the first cross. F1 is the generation made up of their offspring, while F2 is the generation that results when F1 individuals are crossed.


How is a monohybrid cross typically conducted?

A monohybrid cross is a genetic cross that studies a single trait. In a typical procedure, an individual who is homozygous dominant for the trait is crossed with a homozygous recessive individual. Their offspring will then be heterozygotes, or "hybrids," and can be crossed further.

For example, imagine that a parental (P) cross takes place between an RR and an rr organism. All offspring will have an Rr genotype, and crossing these offspring further will yield a 3:1 dominant-to-recessive ratio of phenotypes according to Mendelian prediction.


Describe the proper setup of a Punnett square for a single-trait cross.

  1. Draw a large square divided into four smaller quadrants.
  2. Along the top of the Punnett square, write the first letter of the first parent's genotype above the left-hand column. Write the second letter above the right-hand column.
  3. Along the left side of the square, do the same for the other parent's genotype, now with one letter corresponding to each row.
  4. Fill in the smaller boxes with the corresponding letters - one from the top of the box, one from the left side.
  5. Each quadrant now contains a potential genotype for the offspring.


A condition that causes dwarfism displays incomplete dominance. BB individuals grow to normal height, Bb individuals have short limbs, and those with the bb genotype do not survive long after birth. Using a Punnett square, determine the probability that two parents with dwarfism will have a bb baby as their first child.

The probability is 25%.

Each parent has dwarfism, meaning that they are both heterozygous (Bb). The Punnett square below shows that a cross of two heterozygous individuals has a 1/4 chance of producing homozygous recessive offspring.


What individuals are involved in a dihybrid cross?

A dihybrid cross involves two separate traits. Specifically, both parents must be dihybrids, or heterozygous for both traits being observed.

Often, a dihybrid cross is preceded by crossing two strains that are pure-breeding (homozygous) for different traits. For example, a cross of AABB and aabb parents will yield 100% AaBb offspring, which are dihybrids and can be further crossed.


An organism with the genotype GgAA is crossed with a ggAa partner. What fraction of their offspring will have a genotype that matches one of the parents?


As this Punnett square shows, the offspring have a 50% chance of having either a GgAA or ggAa genotype.


What is a test cross used to determine?

A test cross is used to find the genotype of an individual with a dominant phenotype. The unknown organism is crossed with a homozygous recessive individual. If any of the offspring are recessive, the unknown parent must be heterozygous.

A test cross would be unnecessary for an organism with a recessive phenotype. Such individuals can only have one genotype: homozygous recessive.


What does the ratio “3:1” signify?

Assuming complete dominance, 3:1 reflects the ratio of dominant to recessive phenotypes in the offspring of a monohybrid cross.

For example, consider a cross between two heterozygous (Rr) organisms. Of the offspring, ¼ will be RR and ½ will be Rr, combining for a ¾ chance of exhibiting the dominant phenotype. Only the ¼ with the rr genotype will display the recessive phenotype.


What does the ratio "9:3:3:1" signify?

Assuming complete dominance, 9:3:3:1 reflects the ratio of phenotypes obtained in a dihybrid cross.

Of every 16 offspring, 9 will display both dominant phenotypes. 3 will display one (Trait A) but not the other (Trait B), while an additional 3 will display Trait B but not Trait A. 1 individual of 16 will exhibit both recessive phenotypes.


Do all inherited traits follow Mendel's two laws?

No, many inherited traits do not follow Mendel's laws.

Phenomena that can cause traits to deviate from Mendel's laws include gene linkage, polygenic inheritance, and extranuclear inheritance. Such traits fall under the general umbrella of "non-Mendelian inheritance."


Explain incomplete dominance and give an example.

Incomplete dominance is an inheritance pattern in which the heterozygous phenotype is a blend of the two homozygous traits.

The most familiar example of incomplete dominance involves flower color. In this pattern, if RR individuals display red coloring and rr individuals are white in color, then Rr flowers would be expected to be pink.


The gene for height in a wild horse species exhibits incomplete dominance, with tall, medium, and short phenotypes. When a tall horse is crossed with the offspring of a tall and a short horse, what potential phenotypes can result?

Offspring from this cross can display either tall or medium phenotypes.

Use T to denote the "tall" allele and t to denote the "short" one. The tall parent must have a genotype of TT. Though the genotype of the second parent is less obvious, it results from the cross of a tall (TT) and a short (tt) horse. The second parent, then, must have a Tt genotype. Crossing a TT and a Tt horse can only result in two phenotypes: the same as those of the parents, tall (TT) and medium (Tt).


Explain codominance and give an example.

Codominance is an inheritance pattern in which two alleles contribute equally to an individual's phenotype.

On the AP Biology exam, the most common example of codominance occurs in blood typing, where three alleles (A, B, and O) exist. Both A and B are dominant over O, and both will be expressed simultaneously in an AB individual.


A woman is heterozygous for blood type but displays the type A phenotype. If this woman has a child with an AB man, what is the probability that this child will carry two dominant alleles with regard to blood type?

The probability is 50%.

Heterozygous individuals with type A blood have a genotype of AO, with A being dominant over O. The cross described in this question is pictured below. The child has a 50% chance of being either AA or AB (two dominant alleles), leaving a 50% probability of either AO or BO (one dominant and one recessive allele).


In dogs, the C locus determines the extent to which melanin is expressed. Possible alleles at this locus include C (full expression), Cb (gray fur), and c (albino), among others. What inheritance pattern does this trait exemplify?

This is an example of multiple allelism, in which three or more alleles exist for each a particular trait. Note that a normal individual still cannot possess more than two alleles per locus.