Mills Biology

California State Standards  2.c Students know how random chromosome segregation explains the probability that a particular allele will be in a gamete  2.d Students know new combinations of alleles may be generated in a zygote through the fusion of male and female gametes  3.a Students know how to predict the probable outcome of phenotypes in a genetic cross from the genotypes of the parents and mode of inheritance.  3.b Students know the genetic basis for Mendel’s laws of segregation and independent assortment.

5 Essential Questions:  Who do we inherit our genetic traits from, and how is this information passed to us?  What are dominant and recessive traits? Examples of each?  What is an allele?  What are the differences between genotypes and phenotypes?  Why do our genetic traits depend on meiosis?

We already know…  Sex cells, or gametes (sperm and egg) are created in meiosis.  The result of meiosis is haploid cells with HALF the normal # of chromosomes (23).  When the haploid sperm cell and haploid egg cell meet, they will make a diploid cell with 46 chromosomes. This is the beginning of YOU.

Sex cells are really important…  Because their genetic information controls what traits kids will inherit from their parents.  Chromosomes contain genes, which control traits. We get 23 chromosomes from mom, and 23 from dad.

Some inherited traits are….

Traits are controlled by genes  The genes for traits are located on the pairs of chromosomes we get from mom and dad.  Some of these genes control the same trait (eye color) but may be different versions of that trait (blue, brown, green eyes)  Different versions of the same trait are called alleles.

Locus for eye color Allele for brown eyes Allele for blue eyes

Genetic Traits  Traits can either be dominant or recessive (brown eyes are dominant, blue eyes are recessive)  We write the alleles for traits as letters.  Dominant traits are written as a capital letter and recessive traits are written as lowercase letters.  B=brown eyes and b= not brown eyes.

 DOMINANT TRAITS  Will be the physical trait no matter what other allele they are paired with.  They are written as a capital letter.  What are some dominant traits??  RECESSIVE TRAITS  Are the physical trait only when paired with another recessive allele.  They are written as a lowercase letter  What are some recessive traits??

Allele Pairs  HOMOZYGOUS  Two of the same alleles for a trait  (BB, bb)  HETEROZYGOUS  Two different alleles for a trait  (Bb)

Expression of traits  Genotype  An allele pair (BB, Bb, bb)  We can’t always tell the genotype from the physical trait.  Phenotype  The outward expression of the allele  Hair color, height, left/right handed, nose shape, # of arms

How do we know this?  Gregor Mendel (1850’s) the “father of genetics”  Began experimenting with pea plants. He wanted to see how traits were passed from generation to generation of plants.  He wanted to see what would happen if he crossed two plants with separate traits (like color – yellow and green)  He determined that there must be two forms of each trait in the plants, that is controlled by a factor.  We call that “factor” an ALLELE.

Genetics using Punnett Squares

Last but not least…  There are two genetic laws that we need to know:  The Law of Segregation: Two alleles for each trait separate during meiosis. During fertilization, two alleles for that trait unite.  When alleles from the parents unite to form a heterozygous allele pair, this is called a hybrid.

 The Law of Independent Assortment: Random distribution of alleles occurs during gamete formation (meiosis), because chromosomes sort independently.  In other words, alleles mix up randomly when chromosomes split in meiosis.

What is a Punnett Square?  A tool that helps us predict the possible children that could come from two parents.  It shows the allelic combinations of offspring

How do we use a Punnett Square? Make a 4-box grid BbBb B b Put one parent’s genotype on the top. Then put the other parent’s genotype on the left.

How do we use a Punnett Square?  After labeling the genotypes of the parents, fill in the rest of the grid.  Write capital letters FIRST B b BbBb BB Bb bb

Probability  After we fill out the Punnett Square, we can figure out the probability of offspring.  Example: 1:2:1  Example: 3:1  Example: 4:0  Because there are four squares, every square is 25% of the probability.

 Monohybrid Cross: A Punnett Square that only focuses on 1 trait. We have only looked at monohybrid crosses.  Dihybrid Cross: A Punnett Square that looks at 2 traits at a time.

Complex Inheritance and Human Heredity

 Recessive Genetic Disorders  A person will only have these disorders if they are homozygous for the disorder (pg. 297)  Cystic Fibrosis  Albinism  Tay-Sachs disease  Sickle Cell Anemia  Galactosemia  A carrier is a person who is heterozygous for the trait (they carry it but do not have the disorder)

 Dominant Genetic Disorders  People who are heterozygous for these disorders will have them.  Huntington’s Disease  Dwarfism (Achondroplasia)  Marfan Syndrome

Incomplete Dominance  When a heterozygous phenotype is in the middle of two homozygous phenotypes.  Example: A homozygous red plant (RR) crossed with a homozygous white plant (rr) will be pink (Rr)

Co-dominance  In Co-dominance, both alleles in a heterozygous allele pair are expressed in the phenotype, instead of just the dominant one.

Sex – Linked Traits  Males have one X chromosome, and females have two.  Sex-linked traits are located on the x chromosome  Red-green color blindness (page 307)  Hemophilia

Nondisjunction  When chromosomes don’t separate correctly in meiosis.  Page 312  In the child, this can result in an extra chromosome, or not enough chromosomes.  Example: Down Syndrome