Biology Unit 9.3: Monohybrid Inheritance

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Unit 9.3: Monohybrid Inheritance

1. Define the terms genotype, phenotype, homozygous, heterozygous, dominant, and recessive.

Genotype: the genetic makeup of an organism in terms of the alleles present.

Phenotype: the physical or other features of an organism due to both its genotypes and its environment.

Homozygous: having two identical alleles of a particular gene (e.g. TT or gg). Two identical homozygous individuals that breed together will be pure-breeding.

Heterozygous: having two different alleles of a particular gene (e.g. Tt or Gg); not pure breeding.

Dominant: an allele that is expressed if it is present (e.g. T or G)

Recessive: an allele that is only expressed when there is no dominant allele of the gene present (e.g. t or g).

Alleles are different forms of the same gene. For example, a certain gene controls eye colour but there are many eye colours out there. If you inherit two different alleles for a particular gene, one may be stronger than the other; the stronger one is the dominant allele while the weaker one is the recessive allele. The effect of the recessive will be masked by the dominant. Dominants are represented with capital letters (e.g. B for a dominant brown allele controlling eye colour) and recessives with lower case letters.

Following on with the eye colour situation: you've got a mother with brown eyes (Bb genotype (B for the dominant brown allele, b for the recessive blue allele)) and a dad with blue eyes (bb genotype). Like with multiplication, you can use a punnet square to calculate outcomes.

	b	b
B	Bb	Bb
b	bb	bb

Let's use the eye colour problem again to describe heterozygotes and homozygotes. If an individual ended up with an Bb genotype, they are known to be heterozygotes because they have two different alleles that make up their eye colour. The individual with the bb genotype is known to be a homozygote because they have the same kind of allele that makes up their eye colour.


2. Calculate and predict the results of monohybrid crosses involving 1:1 and 3:1 ratios.

To understand this concept let's use an example involving flowers, because flowers are nice. We can take two red flowers and predict their breeding outcomes. One flower's genotype is RR, while the other is Rw; the latter flower contains a recessive white allele.

	R	R
R	RR	RR
w	Rw	Rw

The ratio of a heterozygous flower to a homozygous flower is 1:1. 

Say you take two purple flowers and and want to breed them to produce a flower. The genotype of the purple flowers would be Pw, where the dominant allele is purple and the recessive allele is white.

	P	w
P	PP	Pw
w	Pw	ww

From the punnet table we see that the possibility of producing a purple flower is 3/4, while producing a white flower is 1/4. The ratio is 3 purples : 1 white.

Biology Unit 9.1+2: Chromosomes and Genes, Cell Division

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9.1: Chromosomes and Genes

1. Define inheritance as the transmission of genetic information from generation to generation.

Well, there you go! But here's a little more detail: inheritance is how parents pass on their characteristics to their children or offspring.

2. Define the terms chromosome, gene, and allele.

Chromosome: A thread of DNA, made up of a string of genes.

Gene: A length of DNA that is the unit of heredity and codes for a specific protein. A gene may be copied and passed on to the next generation. 

Allele: Any of two or more alternative forms of a gene.

3. Define the terms haploid nucleus and diploid nucleus.

Haploid nucleus: a nucleus containing a single set of unpaired chromosomes.

Diploid nucleus: A nucleus containing two sets of chromosomes.

A set is 23 single chromosomes, meaning a haploid nucleus has 23 chromosomes while a diploid nucleus has 46 (who pairs of 23). Body cells (somatic cells) are known as diploid nucleuses, while male and female gametes are haploid nucleuses. 

When an egg and sperm fuse in the process of fertilisation, the 23 chromosomes from the egg and the 23 chromosomes from the sperm add up together in the zygote, returning the chromosome number to 46. (Each gamete is a h______ n______; the zygote is a d______ n______)

Think about it this way:
Haploid sounds like half;
Diploid means 'double' or 'two'

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4. Describe the inheritance of sex in humans (XX and XY chromosomes)

All eggs contain an X chromosome while sperm contain either X or Y chromosomes. When the X egg is fertilised with an X sperm, the XX chromosome makeup makes a girl. When the X egg is fertilised with a Y sperm, the XY chromosome makeup makes a boy. Therefore the baby has a 50/50 chance of being a girl or boy, and it is the sperm that determines the baby's gender. 

9.2: Cell Division

1. Define mitosis

Mitosis: nuclear division giving rise to genetically identical cells in which the chromosome number is maintained by the exact duplication of chromosomes.

Mitosis begins with a single cell, so it contains 46 chromosomes (23 pairs). This is known as the diploid parent cell. The cell makes a copy of each chromosome, resulting in 92 chromosomes. It then divides itself in half, forming two new cells that each contain a full set of chromosomes and are identical to the parent cell. They are known as the daughter diploid cells.

Gotta love that comic sans!

What type of cells go through mitosis? Body cells! The daughter cells end up with a full set of chromosomes, and we know that body cells are diploid nucleuses.


2. Sate the role of mitosis in growth, repair of damaged tissues, replacement of worn out cells and asexual reproduction.

As said above, mitosis is used to provide body cells that replace old or dead ones. It is also used for asexual reproduction, as asexual reproduction produces a clone of the parent with the exact same genetic makeup.


3. Define meiosis

Meiosis: reduction division in which the chromosome number is halved from diploid to haploid.

Meiosis is similar to mitosis. The process is the same until the end; the two daughter cells split in half again, resulting in four cells that contain 23 chromosomes each.

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4. State that gametes are the result of meiosis.

Gametes are the result of meiosis. *Heh* We know that gametes contain 23 chromosomes each, and meiosis results in four cells that contain 23 chromosomes each. It adds up!

5. State that meiosis results in genetic variation so the cells produced are not all genetically identical.

To form a zygote, you need an egg and sperm. Both of these gametes contain different genetic information, so through meiosis genetic information is shared and split between the cells. This results in genetic variation, where cells don't all contain the same genetic information.

This can be beneficial as if the parent has a disease, it could potentially be passed down to the child; genetic variation makes it so that the disease has a chance of not being passed.