SC.912.L.16.1 Genetics
You need to know how to use Mendel's Laws of Segregation and Independent Assortment to analyze patterns of inheritance.
You need to know how dominant, recessive, incomplete dominance, codominance, sex-linked, polygenic, and multiple allele modes of inheritance cause observed inheritance patterns.
TUTORIALS
TEST Cross and Dihybrid Cross
Complex Modes of Inheritance
EOC PRACTICE QUESTIONS
In the mid-1800s, the Austrian monk Gregor Mendel used pea plants to study how traits are passed from one generation to the next. His ideas became the foundation for the basic principles of genetics.
Dominant and Recessive Traits
Organisms that reproduce sexually received genes from both their parents. Those genes may be the same or different such as yellow or green seeds in pea plants. The different forms of the gene for a specific trait are called alleles.
An organism that receives two identical alleles for a characteristic shows that characteristic. However, when an organism has two different alleles for a trait, the dominant allele is expressed.
A Dominant allele is always expressed in an organism and it is represented by an uppercase letter
A recessive allele is expressed in an organism only when no dominant allele is present. Recessive alleles are represented by lower case letters.
For example, if a plant has both purple and white alleles for flower color but blooms purple flowers, then purple is the dominant form while the white is the recessive form.
An organism that has two dominant or two recessive alleles for a trait is homozygous or purebred (two identical alleles of a particular gene)
An organism is heterozygous or hybrid if it has two different alleles for a trait.
Genotype: Genetic make up of an organism (set of genes carried by an organism)
Phenotype: Physical characteristics of an organism
The genotype of an organism determines its phenotype. The phenotype can also be affected by conditions in the environment, such as nutrients and temperature.
Mendelian Genetics
Mendel used pea plants because they could produce purebred individuals, that is genetically identical organism produced through self-pollination
Mendel crossed purebred green-pod plants with purebred yellow-pod plants. All the offspring had green pods because the parent plants were homozygous. The green-pod pea plant had two alleles for green (GG) and the yellow-pod pea plant had two alleles for yellow pods (gg). The cross produced heterozygous offspring, each of which had an allele for green pods and one allele for yellow pods(Gg)
A Punnett square is useful for finding the results of a genetic cross
Making a Punnet Square
One Factor Cross Results
3/4 or 75% large beaks
1.4 or 25 % short beaks
Ratio 3:1
Two Factor Cross Results
9/16 tall plants with green pods
3/16 tall plants with yellow pods
3/16 short plants with green pods
1/16 short plants with yellow pods
Ratio 9:3:3:1
Summary of Mendel's Principles:
1. Inheritance is determined by units called genes, which are passed from parents to offspring.
2. Some alleles are DOMINANT and some are recessive. Dominant alleles are always expressed. Recessive alleles are expressed only when no dominant alleles are present.
3. For each inherited character, an individual has two copies of each gene—one from each parent. These genes segregate from each other when gametes are formed. Thus, gametes carry only one allele for each inherited character.
4. Alleles for different genes usually segregate independently of each other.
The Law of Segregation: The two alleles for a single gene segregate (separate) when gametes are formed.
The Law of Independent Assortment: The alleles of different genes separate independently of one another during gamete formation
Alleles are distributed to sex cells independently of one another. This can be illustrated using dihybrid crosses.
Other Patterns of Inheritance
Most genes do not behave quite so neatly as the two-allele pattern of simple dominance shown by Mendel's peas. Many genes are quite more complicated
Incomplete Dominance
Some alleles are neither dominant nor recessive.
Incomplete dominance: One allele is not completely dominant over another.
.
In Four o'clock plants the allele for red and white flowers show incomplete dominance. Heterozygous (Rr) plants have pink flowers
Codominance
Both alleles are clearly expressed.
Examples: chicken feather color, human protein controlling blood cholesterol levels
.
The roan coat color in cattle is an example of codominance.
If a white cow is crossed with a red cow, the offspring will have a coat made of both red and white hairs.
Multiple Alleles
A gene with more than two alleles
As an example, let’s consider a gene that specifies coat color in rabbits, called the C gene. The C gene comes in four common alleles and they have different levels of dominance.
Polygenic Inheritance
Traits controlled by two or more genes are said to be polygenic traits.
At least two and as many as a dozen of genes are responsible for the many different shades of human eye color. Polygenic traits typically show a wide variety of phenotypes.
Examples: skin color, eye color and height in humans are controlled by the action of many pairs of genes.
A genome is the full set of genetic information that an organism carries in its DNA.
A karyotype shows the complete diploid set of chromosomes grouped together in pairs, arranged in order of decreasing size.
Types of Chromosomes
AUTOSOME any chromosome that is non a sex chromosome
Allosome SEX CHROMOSOMES X and Y.
Females have two X chromosomes
Males have one X and one Y chromosome
.Females are XX and males are XY
More than 1400 genes
are found on the human
X chromosome.
The human Y chromosome contains only about
158 genes.
Sex Linked Traits
Sex Linked Traits are determined by alleles that are located on the sex chromosomes.
Most sex linked traits are recessive and carried on the X chromosome.
Because males are have only one X chromosome, a male who carries a recessive allele will show the sex-linked recessive trait. By contrast, a female must inherit two recessive alleles for the recessive trait to be expressed.
Examples: Hemophilia, Muscular Dystrophy and Colorblindness
Hemophilia is a genetic sex linked recessive disorder in which blood doesn't clot normally because it lacks sufficient blood-clotting proteins
The punnet square shows a a cross between a normal carrier female and a normal male
None of their daughter will be affected and if they have a boy the chances of having hemophilia is 50%
Calico cat
In cats, for example, a gene that controls the color of coat spots is located on the X chromosome.
One X chromosome may have an allele for orange spots and the other X chromosome may have an allele for black spots.
Male cats, which have just one X chromosome, can have spots of only one color. Therefore, if the cat’s fur has three colors—white with orange and black spots, for example—you can almost be certain that the cat is female.