Jan 14th, 2020n We’re getting back into testing practice

with the Science Practice Testn Lesson on Geneticsn Practice with Punnett Squares

– By the end you should be able to identify key words associated with genes, DNA, etc.

– Using Punnett Squares to follow inherited traits

n Time to sign up to tests, or GED/HISET website

Genetics Using Punnett Squares

Early Genetics• The study of genetics

began with observations made by Gregor Mendel.

• After noticing that the flowers his pea plants were either violet or white, Mendel began to study the segregation of heritable traits.

Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants.

Remember that Mendel worked almost 150 years ago when nobody knew about genes or even the structures (chromosomes) that carry genes.

Here are some traits observed by Mendel:

Lets consider a single gene…• A gene carries

information that determines your traits.

Traits are characteristics you inherit from your parents.

• Genes are located inchromosomes.

• Chromosomes come in pairs and there are thousands, of genes in one chromosome.

Continued…• In humans, a cell’s nucleus

contains 46 individualchromosomes or 23 pairs of chromosomes.

• Half of the chromosomes come from one parent and half come from the other parent.

This is a human karyotype representing the 23 pairs of chromosomes in a male Here is the detailed

structure of a chromosome

MULTIPLE BIRTHSn ZYGOT: the cell that is formed when a sperm

fertilized an egg (ovum)

n MONOZYGOT: Identical Twins 1 Egg + 1 Sperm– Fertilized ovum splits into 2 identical cells - Always the same

gender

n DIZYGOT: Fraternal Twins 2 Eggs + different Sperm– Will look different - May be different or the same gender

n MULTIPLE BIRTHS: More than 2 – May be identical, fraternal or both - May be different or the

same gendern CONJOINED (Siamese) TWINS: Ovum splits apart, but the

separation is not completed. Babies are joined at some part of their bodies.

Definitions• Allele- discrete version of the same gene • Genotype- the genes of an organism for one

specific trait • Phenotype- the physical appearance of a trait in

an organism

Definitions• Dominant trait refers to a genetic feature

that “hides” the recessive trait in the phenotype of an individual.

• The term "recessive” describes a trait that is covered over (or dominated) by another form of that trait and seems to disappear.

• Homozygous= two alleles that are the same for a trait (Pure)

• Heterozygous= two different alleles for a trait (Hybrid)

Some other detailsn DNA is your body’s internal, hard-coded

instruction guide for making you, youn DNA is found in your cell’s nucleus, and

you get half from each parent.n Genes are DNA sequences that contain

complex instructions.n Just 1 percent of your DNA sequence is

genes.

Practice• We use two letters to represent the genotype.

A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele).

• Example below: P=dominant purple and p= recessive white

The phenotype for this flower is violet while its genotype (if homozygous) is PP.

The phenotype for this flower is white while its genotype is pp (to be white the flower must have two of the recessive copies of the allele).

Punnett Squaresn The Punnett square is

the standard way of working out what the possible offspring of two parents will be. – It is a helpful tool to

show allelic combinations and predict offspring ratios.

Before we go further lets review how to set up a Punnett Square…

We begin by constructing a grid of two perpendicular lines.

Next, put the genotype of one parent across the top and the other along the left side.

For this example lets consider a genotype of BB crossed with bb.

B B

b

b

• Notice only one letter goes above each box

• It does not matter which parent’s genotype goes on either side.

Next, fill in the boxes by copying the column and row head-letters down and across into the empty spaces.

B B

b B

B

B

Bb

b

b

b

b

Punnett Squares

nNow that we have learned the basics of genetics lets walk through some examples using Punnett Squares.

Lets say:W- dominant white w- recessive violet

W wW

Parents in this cross are heterozygous (Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have violet flowers? ANSWER: 25% (homozygous recessive)

Usually write the capital letter first

w

W W W w

W w w w

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond.

Rr rr

Rr rr

R rr

r

What percentage of the offspring will have red hair? 50%

Let’s try some more…In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant and a

short pea plant.

Tt tt

Tt tt

T t

tt

What are the percentage of phenotypes?50% tall 50% short

Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat

with black eyes and a rat with red eyes.

Rr Rr

Rr Rr

R Rrr

What is the possibility of a red eye off springs?

0% L

Referencesn http://www.athro.com/evo/gen/punnett.htmln http://www.kidshealth.org/kid/talk/qa/what_is_gene.htmln http://brookings.k12.sd.us/biology/ch%2011%20genetics/punnettpr

actice.ppt#1n http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sci

ber/punnett.htmn http://www.biotechnologyonline.gov.au/images/contentpages/karyot

ype.jpg