Biology Unit 3 Explanation & Notes

Section 6.3 Mendel and Heredity – “The father of genetics” KEY WORDS: traits, inherit, discrete, purebred, cross, law of segregation

Traits – distinguishing characteristics that are inherited. Examples include: eye color, leaf shape, and tail length.

Who is Mendel? Gregor Mendel – a monk in the mid 1800s who by experimenting with plants, discovered that traits (genes) are inherited (passed on to) as discrete (separate) units from the parents. Each organism gets two copies of each discrete unit, one from each parent.

What did he do? Used pea plants because they reproduce quickly and he could control how they mate. The flowers on pea plants self-pollinate; so Mendel removed the plant stamens (male reproductive organ) and then took the pollen and fertilized the pistil (female reproductive organ) of another plant. Since he started with purebred (genetically uniform) plants, any changes he saw were based on his experiments.

What did he observe? He picked pea traits that had “either-or” characteristics: shape, color, height, and position. You either saw the plant tall or short and peas were either wrinkled or round. A cross is the mating of two organisms and Mendel crossed purebred purple-flowered pea plants and purebred white-flowered pea plants. The offspring, or F1 generation, all had purple flowers. But, these plants self-fertilized, the next generation (F2) resulted in purple and white flowers!

So what did he conclude? After crossing thousands of plants he saw a pattern in a ratio (comparison how things relate) of the F2 generation. About three-fourths (3/4) of the plants had purple flowers and about one-fourth (1/4) had white flowers. In other words the ratio is 3:1 of purple:white flowers.

From these observations he believed that traits are inherited as separate (discrete) units and not blended or diluted (thinned down) over generations. His law of segregation (Mendel’s first law) states that:

You get two copies of each gene, one from each parent. You donate only one copy of each gene to gametes (eventually your offspring).

**On your vocabulary guide, fill out one box for either the word purebred, cross, or law of segregation – then pair share your result**

Section 6.4 Traits, Genes, and Alleles – “same gene, different version”

A gene is a piece of DNA that provides a set of instructions for a cell to build proteins. Since there are many genes on a chromosome, the locus is the “address” or the location of the gene on the chromosome. Also, you can have different forms of genes, just like you can have more than one kind of car, bread, or ice cream flavor. An allele (uh-LEEL) is one of several forms of a gene that can take place at a specific locus.

If you happen to have to same type of alleles (purple and purple flowers) at the locus, then they are considered homozygous. If you happen to have different types of alleles (purple and white flowers) at the locus, then they are considered heterozygous.

The root word HOMO means ________________ , and the root word HETERO means ____________________.

Genes influences trait development

Unless you have an identical twin, you have a unique collection of genes, or genome that makes up all of your traits. When you refer to just the genetic makeup of certain genes, it is called a genotype. When you refer to the physical characteristics or traits then it is called the phenotype. Take a look at the picture of the eye colors for example.

Notice how brown eyes can have two genotypes? That is because the brown eye allele is dominant or expressed when two of them are present or matched with a different allele. That different allele would be the blue eye allele. The blue eye allele is recessive because it is only expressed when it is matched with another blue eye allele. A dominant allele will always mask the recessive allele. Ever been out with a friend and you are trying to decide where to eat? How would a dominant and recessive pair of friends discuss where to go to eat? How would two recessive friends decide?

Individual letters are used to represent alleles. For example, the brown eye allele is B and the blue allele is b.

Why is the brown eye allele upper case? _________________ Why is blue eye allele lower case? __________________

**Pair up! Complete pages 10-11 in your Biology Unit 3 SAW**

Section 6.5 Traits and Probability – Punnett squares shows the cross

When it comes to predicting genes, math plays a big part. That’s why the Punnett square was made as a grid system to predict all possible genotypes that result from cross so that the math could be seen!

One trait crosses

So far, we’ve looked at crosses that show the inheritance of only one specific trait like the flower color. These are monohybrid crosses. Mono means: ______________!

Fill in the following blanks that match the allele types in the Punnett squares below:

________zygous - __________zygous ________zygous - __________zygous ______zygous - _________zygous

The ratio of the genotype of the 2nd Punnett square can be written as – : :

The ratio of the genotype of the 3rd Punnett square can be written as – :

Two trait crosses

A cross that examines the inheritance of two different traits, like the color and shape of a pea pod, is called a dihybrid cross. DI means __________!

This is a heterozygous-heterozygous cross of peas. What are the two separate traits that Mendel observed in this dihybrid cross?

He observed pea _____________ and pea _____________.

How many different genotypes and phenotypes are there?

Total genotypes: _______ Total phenotypes: ________

What is the phenotypic ratio? ____ : _____ : _____ : _____

Mendel used this dihybrid cross to conclude that the presence of one trait did not affect the presence of another trait. His law of independent assortment states that allele pairs separate independently of each other (during meiosis); different traits are inherited separately.

In meiosis, the gametes can either inherit a dominant or recessive allele, it is random. The likelihood of a particular event happening is called probability and the probability of a certain allele being passed on can be determined. Example, you flip a coin, what’s the chance of getting tails?

Making a Smiley Face with Traits!

B B

b

b

B b

B

B

B b

b

b

Instructions: In pairs, get two coins from your teacher. Mark one “F” and the other “M” to represent the parents. The parents are heterozygous for all smiley face traits. Heads is dominant, tails is recessive. Flip the coins for each trait and then circle the correct dominant or recessive letter. Use the smiley face traits key to find the genotype and phenotype.

To determine whether your smiley face is a male or female, flip the male parent coin. Heads is X and tails is Y.

Now draw your smiley face with all of the traits you created! Don’t forget to name your smiley face.

Section 6.6 Meiosis and Genetic Variation (complete p 24-25 #1-8; skip rest on page 25)

We have discussed how genetic diversity makes humans look different from each. This is mainly because of the independent assortment (Mendel!) of chromosomes during meiosis and random fertilization. This means that parents can create over 70 trillion children with a unique set of chromosomes!

To mix things up even more, crossing over occurs which is the exchange of chromosome segments between homologous chromosomes during prophase I of meiosis I. Take a look at the diagram to the right. How can you model crossing over with your hands?

Section 7.1 Chromosomes and Phenotype – How disorders carry on

There are many disorders that can cause severe diseases or even death yet they continue to get passed on to offspring. Why is this? Why doesn’t the disorder die with the individual?

That is because sometimes, disorders can be masked or hidden as a recessive allele. A carrier is someone who does not show the symptoms of the disease but can pass on the disease-causing allele.

This is often the case with sex-linked disorders. Sex-linked genes are genes located on the sex chromosomes (X and Y); for females it’s XX and male it’s XY. A male is more likely to sex-linked disorder because there only one copy of each gene in the XY chromosomes so genes with disorders will have a harder time being masked.

Section 7.2 Complex Patterns of Inheritance – blending and spotting

Incomplete dominance (blend) - a heterozygous phenotype that’s between the two homozygous phenotypes; neither completely dominant nor completely recessive – ex. of red and white plant color making a pink plant

Codominance (share) – traits that are fully and separately expressed by alleles that are not dominant nor recessive – ex. is a red plant crossing with white plant to make a red and white plant

Polygenic traits – traits that are made by two or more genes – ex. is human skin color comes from four genes that together make one skin color

Section 7.4 Human Genetics and Pedigrees

Sometimes, a couple wants to know their child’s chances of having a genetic disorder. A pedigree is a chart that helps trace the phenotypes and genotypes in a family to find out who is a carrier of recessive alleles. On a pedigree, boxes represent males and circles represent females. A blank shape shows the person is not a carrier while a colored shape shows that the person is. A half colored shape shows the person is a carrier.

PEDIGREE EXAMPLE:

One method scientists use to study chromosomes is to look at their patterns. Karyotypes are pictures of all the chromosomes in a cell that are made by a chemical stain and it can show changes in the chromosomes.

KAROTYPE EXAMPLE:

** Complete P. 36 in your unit 3 SAW!