Biology, Answering the Big Questions of Life/genetics1

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Why do children look like combinations of their parents?[edit]

Children often look like some combination of their parents. This is because each parent gives the child some of themselves {gene}. A child is made from the information found in the cells of the parents. These characteristics are called genes.

The traits that you can observe in a person are called that person's phenotype. The information that makes a person look the way they do is encoded in structures called chromosomes that are found in their cells. The specific genes found on these chromosomes make up what is called that person's genotype.

In humans, each person has two copies of each type of chromosome. That means they have two copies of each gene, but a person will give only one of these genes to their child. For example, a person could have a gene for brown hair and a gene for blond hair. He would give one of those traits to his child. The child's phenotype would depend on what gene he received from his other parent.

So children look like combinations of their parents because they are. Each parent gives half of their genetic material to their children. The combination makes a unique combination of their parents genes.

The scientific study of how traits are passed from parents to children is called genetics.

Chromosomes in the nucleus of the cell contain information of inheritance of features from parents to next generation in the form of DNA (Deoxyribo Nucleic Acid) molecules

Who discovered modern Genetics?[edit]

A monk named Gregor Johann Mendel who lived in the early to mid 1800s in the town of Brünn (now part of the Czech Republic) is considered the founder of modern genetics.

Gregor Mendel had studied statistics in Vienna and he wanted to bring some of the mathematics of that discipline to the study of plant breeding. He grew sweet pea plants and counted the properties of the children of hybrid crosses through generations to try to discern the rules that governed the transfer of properties from parent to child.

Why sweet peas?[edit]

Because sweet peas had clear unambiguous traits such as flower color and seed color. These traits made it easy to get good data.

What did Mendel discover[edit]

Mendel's First Law - The law of segregation This law states that each parent has two copies of each trait (gene), but they only give one trait to their offspring and which one they give is random.

When one uses symbols to show the passage of traits, then one easily gets the 3:1 ratio that Mendel found in the second generation when he crossed true-breeding plants.

Mendel's Second Law - The law of independent assortment If you look at more than one trait such as coat color and flower color, these traits will sort independent of each other. The chance of getting one or the other coat color gene is 50/50 and the chance of getting one or the other flower color gene is also 50/50, therefore the ratio of the combinations of the two traits are decided purely by chance.

What do those strange terms mean?[edit]

Phenotype - An expressed gene, what an organism looks like.

Genotype - What is in the genes. Which alleles and organism has.

Allele - Two or more different forms of the same trait such as blue eyes and brown eyes.

What's a genetic cross?[edit]

A Genetic cross is a way of predicting the traits of the offspring of two parents.

A. Monohybrid cross - A monohybrid cross is a cross looking at only one trait at a time. If the parents are true-breeding, there will be a 3:1 phenotypic ratio in F2.

B. Dihybrid cross - A Dihybrid cross is a cross looking at two traits at a time. If the parents are true-breeding, there will be a 9:3:3:1 phenotypic ratio in F2.

C. A Test cross (Also called a backcross)- Is a cross with the homozygous recessive parent. It reveals the genotype of an individual with a dominant phenotype.

How does one solve a genetics problem correctly?[edit]

Follow these steps: 1. Write down the phenotype of the parents.

2. Write down the genotype of the parents.

3. Write down the gametes that each parent can give.

4. Show all possible combinations of these gametes. (A Punnett square makes this easy).

5. Figure out the phenotype for each of the genotypes shown in the square.

6. Count the number of organisms with each phenotype and write a ratio.



Relationship between Genotype and Phenotype. Chromosomes are where genes reside.


Biology, Answering the Big Questions of Life/genetics2


EXERCISE • genetics1 • Problems
  1. If we do not know if a purple flower is homozygous or heterozygous. How can we tell?
SOLUTION • Biology, Answering the Big Questions of Life/genetics1 • Problems
  1. Perform a "text cross" with the white flower (homozygous recessive).