1. Mendelian Genetics

2. Learning Targets
Dominant alleles mask the effects of recessive alleles but co- dominant alleles have join effects
Many genetic diseases in humans are due to recessive alleles of autosomal genes, although some genetic disease are due to dominant or co-dominant alleles
Some genetic diseases are sex-linked. The pattern of inheritance is different with sex-linked genes due to their location on sex chromosomes

3. Vocabulary terms Genes Genotype Diploid Phenotype Haploid Polygenic
Mutation
Continuous variation
Alleles
Codominance
Dominant
Incomplete dominance
Recessive
Multiple alleles
Hybrids
Homozygous dominant
Homozygous recessive
Heterozygous
Do these human traits follow a simple Mendelian inheritance pattern or are they polygenic?
Widow’s peak
Hitchhikers thumb
Earlobe shape
Dimples
What would the data look like to support either possibility?
The vocabulary words listed in the slide are used throughout this unit. Make sure that you understand what they mean and how to use them correctly.
Mendelian genetics is a pattern of inheritance in which there is only one gene that controls a trait, and only a few alleles for that gene. You may have learned examples of “dominant” and “recessive” alleles in the past. These are referring to Mendelian genetics.
Mendelian genetics is a simple form of genetics to understand, but most traits are not controlled by only one gene with only two alleles. Most traits are controlled by many genes, with environmental factors controlling how those traits are expressed. Polygenic inheritance for a trait, therefore, is way more common than simple mendelian inheritance patterns. As we learn the basics of genetics, keep this in mind.

4. Patterns in Inherited Traits
Gregor Mendel, an Austrian Monk, gathered the first experimental evidence of the genetic basis of inheritance.
Between 1856 and 1863 Mendel cultivated and tested some 29,000 pea plants (Pisum sativum).
Mendel showed evidence that genes are physical particles (units) that become segregated during gamete formation and that units specifying one trait (e.g. flower color) sort independently of units specifying other traits (e.g. seed shape).
Mendel’s work is important today in genetics, but when he completed his research and published his results, he received no recognition for what he did. In fact, Mendel died (1884) before the significance of his work was recognized in 1900.

5. Mendel’s Method Parent – the P generation
First filial offspring – the F1 generation
Some terminology common to genetics. The P generation = “parent generation,” the F1 generation = “first generation,” followed by F2, etc.
F1 seed
F1 plant

6. Mendel’s Results How can we predict these outcomes?
Mendel studied pea plants, which fortunately, have several traits that follow simple Mendelian inheritance patterns. For example, the gene for seed shape has two alleles – either round seeds or wrinkled seeds. The trait for pod color has two different alleles – either green pods or yellow pods.
Mendel called a trait “dominant” if it appeared more often in a population than a “recessive” trait.
Can you notice any similarities in the ratios that Mendel recorded within the F2 generation for each of these traits?

7. Genotype vs. Phenotype
Genotype refers to the genetic makeup of an organism. For example: Aa or TtPp are examples of genotypes.
Phenotype refers to the set of observable characteristics resulting from its genotype and environment. For example: Purple vs. white
phenotype
genotype

8. Segregation and recombination during meiosis
Alleles become segregated during gamete formation. This is now called Mendel’s Law of Segregation.
Mendel explained his results in terms of two laws: the Law of Segregation and the Law of Independent Assortment. I think it’s important to note that Mendel didn’t know about gene or DNA. This was in the 19th century. He only hypothesized that there must be some sort of unit that was passed down from parent to offspring which conveyed particular traits.
His Law of Segregation stated that organisms must have two versions of each of these units (one from the mother and one from the father), which become separated, or segregated, from each other when packaged into egg and sperm cells. We now know that these units are genes and that the different versions are alleles. We now also know that the process of Meiosis is responsible for segregating these alleles from each other.
Recombination: genetically distinct from either parent.

9. In the hypothetical germs cells above, which are undergoing meiosis, you can see that the parent on the left (dark blue) has two dominant (capital “A”) alleles for the trait and the other parent (light blue) has two recessive (lowercase “a”) alleles for the same trait (the “A” trait). Once meiosis has occurred and gametes are formed, one gamete from each parent can go on to participated in sexual reproduction (see arrows pointing to lower cell). Genetic recombination happens in sexual reproduction. What combination of alleles will all offspring of these two parents receive, regardless of which gametes are involved in sexual reproduction? Why?
What can we predict about the possible outcome of a cross between two parents from this F1 generation?

10. (1st filial generation)
The “P” (parental) Cross
F1 PHENOTYPES
(1st filial generation)
True-breeding
homozygous recessive
parent plant pp
True-breeding
homozygous dominant
parent plant Pp Pp p p P Pp Pp
The purple box in the picture is called a Punnett Square. It is a way to determine probability, or likelihood of genetic crosses. The two parents here are both homozygous for their traits. In other words, they both have two of the same allele. The parent with white flowers is homozygous recessive (has two of the same recessive alleles) and the parent with purple flowers is homozygous dominant (has two of the same dominant alleles). Mendel would have called these parents “true-breeding” because they are the result of breeding only white with white over many generations and breeding only purple with purple over many generations.
The Punnett Square allows us to view the genetic possibilities in all possible gametes – Either P or P for the purple flower and either p or p for the white flower. The boxes show each possible combination of alleles resulting from a cross between these two parents. Because both parents were homozygous for their traits, all potential offspring would have the same combination of alleles – one P and one p. This is called heterozygous (two different alleles). PP P Pp Pp Pp Pp

11. F2 PHENOTYPES and their predicted ratio
The F1, Monohybrid Cross
An F1 plant self-fertilizes
and produces gametes:
F2 PHENOTYPES and their predicted ratio Pp PP Pp P p
If we take the offspring of the parent generation and breed them together, we can observe a classic Mendelian monohybrid cross. (Mono = one, hybrid = composed of more than one type). A monohybrid cross, therefore, is one in which two individuals that are heterozygous (hybrid) for one trait are crossed together. The offspring of the F1 generation are called the F2 generation. Notice that all parents in the F1 generation were purple, but we see the white trait reappear in the F2 generation. A dominant trait covers up or masks the effects of a recessive trait when combined together. Therefore, any parent with at least one copy of the “P” allele will be purple, even if it also has a copy of the “p” allele. In order to express the trait, a plant would need two copies of the “p” allele.
Also make note of the ratio of dominant to recessive traits that are likely to appear in the F2 generation, according to this Punnett Square (3 purple: 1 white). Refer back to Mendel’s results on slide 5. How can you explain those results using the probability you observe here? P PP Pp p Pp pp Pp pp
3 purple: 1 white

12. 3 purple: 1 white predicted outcome
POSSIBLE EVENT:
PROBABLE OUTCOME:
sperm P meets egg P
sperm P meets egg p
sperm p meets egg P
sperm p meets egg p
1/4 PP offspring
1/4 Pp
1/4 pp p P pp
female gametes
male gametes p P pp Pp p P pp Pp p P pp Pp PP
This shows how to use a Punnett Square to determine probabilities. We write the probable gene combinations, or the genotype, of the offspring by using capital letters to represent the dominant trait and lowercase letters to represent the recessive trait. Therefore, PP is a homozygous dominant genotype, pp is a homozygous recessive genotype, and Pp is a heterozygous genotype for the trait of flower color in pea plants.
The genotype determines the phenotype. The phenotype is the trait that is expressed. Therefore, more than one genotype can determine the same phenotype. For example, a plant with a geneotype of PP or a plant with a genotype of Pp will both have a phenotype of purple, since the presence of at least one dominant allele determines the trait. We know that a plant with a recessive phenotype (such as white flowers) must have a genotype of pp.
Notice that the phenotypic ratio of a monohybrid cross, therefore, is 3:1.
3 purple: 1 white predicted outcome

13. The Dihybrid Cross - following two sets of alleles instead of one
A dihybrid cross is one in which both parents are heterozygous (hybrid) for two traits. Figuring out probable outcomes in offspring for two traits becomes more complicated that for one trait.
In the example on this slide, one parent is homozygous dominant for two traits (“A” and “B”), and the other parent is homozygous recessive for the same two traits. Because both parents are homozygous for both traits, all of the F1 individuals from this crossing would be heterozygous for both traits, or dihybrid.
This F1 individual is called a dihybrid and is the product of recombination.

14. Mendel hypothesized that “units” specifying one trait (in this case, A) sort into gametes independently of “units” specifying other traits (B). Data supported Mendel’s hypothesis which is now called the Law of Independent Assortment
Nucleus of a diploid (2n) reproductive cell with two pairs of duplicated homologous chromosomes
Possible alignments
of the two homologous
chromosomes during
metaphase I of meiosis
The resulting alignments
at metaphase II
Allelic combinations
possible in gametes
Here we can see both the Law of Segregation and the Law of Independent Assortment occurring. The Law of segregation is shown when both of the traits are segregated (separated) from each other during meiosis and put into gametes. The Law of Independent Assortment, on the other hand, demonstrates that there is more than one way in which one of the A alleles can be combined with one of the B alleles. What this means is that a plant that is dominant for A could be either dominant or recessive for B. Because these genes are located on different chromosomes, the traits occur independently of each other when expressed in the organism. Remember that this is randomly determined in Meiosis, depending on how the homologous chromosomes line up in Metaphase I and are pulled apart in Anaphase I
Remember linkage groups? When genes are located on the same chromosome, they are not always separated independently from each other. If crossing over occurs, they could be separated from each other, but it is less likely than if they are located on different chromosomes. Almost all of the traits that Mendel studied in peas were located on different chromosomes, so all of his results supported the law of independent assortment.
1/4 AB
1/4 ab
1/4 Ab
1/4 aB
In this example, the A and B genes are not part of a linkage group because they are on separate chromosomes.

15. PpTt PPTT purple- flowered tall parent (homozygous dominant) pptt
white-
flowered
dwarf parent
(homozygous
recessive)
Only possible gametes PT X pt
F1 OUTCOME:
All of the F1 plants are PpTt heterozygotes
(purple flowers, tall stems).
PpTt
Possible outcomes from crossing a pea plant that is homozygous dominant for flower color and stem height (PPTT) with a plant that is homozygous dominant for both traits (pptt). All resulting offspring in the F1 generation will be heterozygous for both traits (PpTt), or dihybrid.

16. 9 purple tall: 3 purple short: 3 white tall: 1 white short
Meiosis and gamete
formation
1/4
1/4
1/4
1/4 PT Pt pT pt
1/4
1/16
1/16
1/16
1/16 PT
PPTT
PPTt
PpTT
PpTt
1/4 Pt
1/16
1/16
1/16
1/16
PPTt
PPtt
PpTt
Pptt
1/16
1/16
1/16
1/16
1/4
PpTT
PpTt
ppTT
ppTt pT
The results of a Punnett Square for a dihybrid cross. If we take the F1 offspring from the example on the previous slide and cross them together (or with themselves), these are all the possible genetic combinations. There are more boxes here because there are more than two ways to combine “P” with “T” in gametes. One gamete could be dominant for flower color and dominant for stem height (PT). Another could be dominant for flower color and recessive for stem height (pt). A third could be recessive for flower color and dominant for stem height (pT) and a fourth could be recessive for both traits (pt). Since both parents were heterozygous, both could potentially contribute these four genetic possibilities in their gametes.
Next, we count all of the different potential genotypes and determine the phenotypes of those geneotypes.
9 will be dominant for both traits. Notice that more than one genotype determines this phenotype, for example, PPTT , PpTT, or PpTt will all be purple and tall.
3 plants will have a genotype that leads to a purple short phenotype (genotypes of either PPtt or Pptt).
3 plants will have a genotype that leads to a white tall phenotype (genotypes of either ppTT or ppTt).
Only one out of sixteen potential offspring will end up with two copies of both recessive traits (pptt) and will therefore display both recessive phenotypes (white and short).
The phenotypic ratio of a dihybrid cross, therefore, is 9:3:3:1.
1/4
1/16
1/16
1/16
1/16
PpTt
Pptt
ppTt
pptt pt
9 purple tall: 3 purple short: 3 white tall: 1 white short

17. Other Patterns of Inheritance – Incomplete Dominance Neither allele dominates the other, and neither allele is fully expressed
Not all genes follow simple Mendelian inheritance patterns. For example, when an individual is heterozygous for a trait, it could express a completely different trait instead of just the dominant trait.
In the case of incomplete dominance, the heterozygous individual displays a blending of the two traits. In this example, the color of the flower is the trait. It could be red (RR) or white (rr). The heterozygous (Rr) individual displays a phenotype that is neither red nor white. It is something in between (in this case pink)… which is why it is called incomplete dominance. Neither color is completely dominant over the other.

18. Other Patterns of Inheritance – Codominance and Multiple Alleles
Codominance occurs when the heterozygous individual expresses both alleles. In human blood type, both the A allele and the B allele are codominant to each other. This means that a heterozygous individual expresses the trait for both A and B blood type, or what is called AB blood type.
Human blood type is also an example of multiple alleles, meaning there are not only two possible alleles for blood type. There are three possible alleles, and therefore several different possible genotypes and phenotypes. The “O” allele is recessive to both the “A” and the “B” alleles. Therefore, if an individual is heterozygous with “A” and “O,” the “A” is dominant and will be expressed. Same for an individual with “B” and “O.” An individual must have two “O” alleles in order to express the “O” phenotype.

19. Reacts (clumps) when RBC’s from groups below are added to blood serum from groups at left.
A B AB O NO
YES
Antibodies Present
Anti-B
Anti-A
A Antigens on surface of RBC’s
B Antigens on surface of RBC’s
A and B Antigens on surface of RBC’s
We use the symbol “I” in blood type. IA is the allele for type A blood. IB is the allele for type B blood. i is the allele for type O Blood.
Possible genotypes therefore, are IAIA and IAi (both type A blood), IBIB and IBi (both type B blood), IAIB (type AB blood), or ii (type O blood).
How does blood type work? Our blood cells have proteins on their surface that have a specific shape and identify the cell as either A or B. These proteins are called antigens because they “fit” with our immune system’s antibodies. Antibodies are proteins produced by the immune system to help recognize threat before they cause harm. When an antibody recognizes an antigen, it stimulates an immune response. (The basis behind vaccines is to stimulate your body to produce and therefore be able to recognize specific antigens on things that could cause you harm, such as bacteria cells, using a dead or weakened form of those cells). If you have type A blood, your body creates type antibodies that recognize and react with type B antigens. Therefore, if you were transfused with type B blood, your body would recognize it as a threat and attack those cells. If you have type B blood, your body produces A antibodies. If you have type AB blood, your body does not produce either antibodies because then your body would attack your own cells. If you have type O blood, your blood cells have neither antigen, but your body produces both the A and the B antibodies. This is why type O people are called “universal donors” but can only accept blood transfusions from other type O individuals. Their blood has no antigens to react with the patient’s antibodies, but if their blood would react with either the A or the B antigens if given a blood transfusion.
No Antigens on surface of RBC’s

20. DBQ Mendelian Genetics

21. Other Patterns of Inheritance – Epistasis one gene controlling the expression of another
Some genes affect the expression of other genes, in what is called epistasis. Albinism is an example of epistasis. The color of a person’s skin is determined by multiple genes, which determine the amount of pigment deposited in the skin. However, the gene that controls the production of pigment must be fully functional in order for the pigment to be deposited in the skin. Albinism occurs when there is a mutation to the gene controlling the production of the pigment (called melanin). This is a case of epistasis because a non-functioning gene for pigment production prevents the genes that control pigment deposition from being expressed. Pigments are deposited in our eyes and hair as well, which is why albinos lack pigmentation of the eyes and hair. An albino’s eyes appear to be reddish in color because of the light reflecting off of the blood vessels in the eyes.

22. Other Patterns of Inheritance – Epistasis one gene controlling the expression of another
Labrador fur color is an other example of epistasis, similar in nature to albinism. There are two genes that control fur color in labrador retrievers. One gene (represented with an “E” in the image) controls whether pigment is produced or not. The dominant form of the gene (E), causes pigment to be produced. The recessive form of the gene (e) does not allow pigment to be produced. A second gene (represented with a “B” in the image), determines how much of the pigment is deposited in the fur. The dominant gene (B) causes higher amounts of pigment to be deposited, resulting in the black fur phenotype. The recessive form of the gene (b) causes lower amounts of pigment to be deposited, resulting in the brown fur phenotype.
An individual must have at least one copy of the dominant E gene and at least one copy of the dominant B gene to be black. (EEBB, EeBB, or EeBb)
An individual must have at least one copy of the dominant E gene and two recessive B genes to be brown. (EEbb or Eebb)
An individual must have at two copies of the recessive E gene to be yellow, regardless of the alleles it posses for the B gene. (eeBB, eeBb, eebb)

23. Other Patterns of Inheritance – Polygenic Traits multiple genes (and often environmental factors) influence a trait and the outcome produces continuous variation
As mentioned at the beginning of this PowerPoint, most traits do not follow simple Mendelian patterns of inheritance. There are usually many genes controlling a trait. This type of trait is called polygenic. Additionally, environmental factors influence how genes are expressed. A child that suffered from malnutrition while developing and growing, for example, may never fully express certain genes. Human height is an example of a polygenic trait because there are multiple genes that determine it, as well as environmental factors. Imagine what it would be like if there was only one gene with two alleles for human height, as Mendel found in pea plants. Some humans would be tall and the rest would be short. There would be no intermediate phenotypes. Generally, in regards to polygenic traits, we observe a range of phenotypes, with most of the population falling somewhere in the middle. This is demonstrated nicely by the images on this slide.

24. Other Patterns of Inheritance – Polygenic Traits multiple genes (and often environmental factors) influence a trait and the outcome produces continuous variation
Chr 15
Chr 19
Eye color is commonly described as a Mendelian trait, but it is, in fact, polygenic. Some of the genes that control eye color, however, do display mendelian patterns of inheritance. For example, you may have learned in the past that brown eyes are dominant to blue eyes. This is because of the bey 1 gene, which has two alleles, brown (B) and blue (b). A second gene, called the gey gene, also has two alleles, green (G) and blue (g). Both the bey 1 gene and the gey gene determine amount of pigment deposited in the eye. The dominant forms deposit more pigment than the recessive forms.
For example, if an individual has at least one copy of the dominant B gene, they will have brown eyes, regardless of which alleles they have for the G gene. If they have two recessive copies of the B gene and at least one dominant version of the G gene, they will have green eyes. An individual must be homozygous recessive for both the B and G genes to have blue eyes.
There are, however, more than two genes involved in eye color, which determine things such as the pattern of pigment distribution (as found in hazel and blue-green eyes). The fact that we can observe a range of traits in regards to eye color tells us that it must be a polygenic trait, regardless of certain mendelian patterns that are followed.
The bey 1 gene on chromosome 15 has two alleles, brown and blue and the gey gene has two alleles on chromosome 19, green and blue. Brown is always dominant to both green and blue, and green is dominant to blue. Amount of melanin protein product (another genetic system) determines eye color intensity. There are more genes involved, but this is the basic mechanism.