Unit 2 Part 2 Bio

TOPIC: HOW CAN WE PREDICT WHICH TRAITS WILL BE INHERITED? (LS.Bio.7.1)

CMS BSCS Alignment: Lesson 10-13

NOTES

ESSENTIAL CONTENT 

• Predicting inheritance of polygenic traits is difficult because of the interaction of multiple genes. For traits that are linked to only one gene, we can make predictions about inheritance if we know the genotypes of both parents.

• Remember that a genotype for a trait consists of 2 alleles.  

  • These alleles are located on chromosomes. 

  • One allele for a trait is carried on a chromosome that was inherited from an individual’s mother and the other allele for a trait is carried on the same type of chromosome that was inherited from an individuals’ father. 

  • For a trait, there are different versions called alleles. There are various types of inheritance that determine how the versions of different traits are expressed in an individual.

  • We can use letters to represent the alleles for a trait. 

How is Cystic Fibrosis related to high cholesterol? 

Draw a Punnett square that shows one parent with a heterozygous genotype (Ff) and the other with the homozygous dominant genotype (FF).

What is the probability that this couple will have a child with Cystic Fibrosis? 

Dominant / Recessive

•  Cystic fibrosis is a disorder that is linked to a membrane protein.  

  • One version of the membrane protein is correctly shaped to regulate the passage of certain materials through the cell membrane.  This is the dominant version of the trait.

    • A capital letter is used to represent the dominant allele (F). 

  • The other version of the membrane protein has a mutation that changes the shape.  This protein is not able to effectively regulate the passage of those materials across the cell membrane, causing an abnormal build-up of thick mucus in the cells of the respiratory tract.  This is the recessive version of the trait.

    • The same letter in lowercase is used to represent the recessive allele (f). 

• The dominant version of a trait can hide or mask the recessive version of the trait when both are present.

• Since an individual has two alleles for each trait (one from the mother and one from the father), the two alleles combined make up that individual’s genotype.

  • If there are two dominant alleles, the genotype is homozygous dominant (FF). Individuals with this genotype have the normal phenotype (no Cystic fibrosis).

  • If there are two recessive alleles, the genotype is homozygous recessive (ff). Individuals with this genotype have Cystic Fibrosis.

  • If there is one dominant and one recessive allele, the genotype is heterozygous (Ff). Individuals with this genotype have the normal phenotype (no Cystic fibrosis) because the dominant allele will mask the recessive allele.

• A Punnett square is a tool that can be used to predict the inheritance of a trait such as Cystic Fibrosis when the genotypes of the parents are known.

  • Example:  A couple is considering having children and they know that Cystic Fibrosis runs in both families.  Neither of them have the disease, but they want to know if they could pass it to their children.  They each have a blood test and find out that they both have the heterozygous genotype (Ff).

  • The Punnett Square shows the possible results of a “cross” between these two individuals.

  • The Punnett Square reveals that there is a 75% chance that a child of this couple will not have Cystic Fibrosis, but there is a 25% chance that the child will have Cystic Fibrosis. 

What is the connection between Tay-Sachs disease and cholesterol? 

Incomplete Dominance

• For some traits, the allele for one version can not completely mask the other allele.  This type of inheritance does not follow the pattern of normal dominance.

• For these traits, there is an intermediate phenotype for a heterozygous individual.  Therefore the alleles for incompletely dominant traits must be represented differently to show this intermediate phenotype.  

• Tay-Sachs disease is a disorder related to the ability to break down certain lipids in the bloodstream. A specific enzyme protein is responsible for breaking down these lipids.  

  • Individuals who have two normal alleles produce the normal amount of enzyme.  Individuals who have two mutated alleles do not produce ANY enzyme and develop Tay-Sachs disease. However, individuals who have one of each version produce half the normal amount of enzyme.

  • The alleles to represent this trait involve using a prime symbol (‘) on one version.

    • EE - Normal enzyme production 

    • E’E’ - NO enzyme production

    • EE’ - Intermediate enzyme production (half)

                            In the case of this disease, individuals with the                    

                            intermediate phenotype DO NOT have Tay-Sachs. The 

                            enzyme produced by the one normal allele is sufficient.

• Example:  A couple is considering having children and they know that Tay-Sachs runs in both families.  Neither of them have the disease, but they want to know if they could pass it to their children.  They each have a blood test and find out that they both have the heterozygous genotype (EE’).

  • The Punnett Square shows the possible results of a “cross” between these two individuals.

  • The Punnett Square reveals that there is a 75% chance that a child of this couple will not have Tay-Sachs.  There is a 50% chance that a child will only produce 50% of the enzyme.  There is a 25% chance that the child will produce no enzyme and have Tay-Sachs.

Which proteins would be “recognized” as normal by the immune system of someone with Blood Type

A?

B?

AB?

O? 

Why are people with Blood Type O considered the universal donors? 

Why are people with Blood Type AB considered the universal acceptors? 

Codominance and Multiple Alleles

• For most traits, there are two different versions or alleles. However, for the human trait of blood type, there are three possible alleles.

• Blood type in humans is determined by proteins on the surface of an individual’s red blood cells.

  • Individuals with type A blood have “A” proteins.

  • Individuals with type B blood have “B” proteins.

  • Individuals with type O blood have no proteins of this type.

  • Individuals with AB blood have both “A” and “B” proteins.

•  The production of these proteins is controlled by a gene. The proteins are a type of antigen called isoagglutinogens.  Antigens are proteins that can be recognized by the immune system.  

  • Individuals with blood types A, B, and AB have the dominant trait of antigen production (I).

  • Individuals with blood type O have the recessive trait of no antigen production (i). 

• Since there are different types of antigens that are produced, we must also indicate which of those is produced by individuals with the dominant phenotype.  For this, a superscript A or B (I^A or I^B) is used.  

• Since there are two possible dominant alleles (I^A or I^B) and one possible recessive allele (i), blood type is referred to as multiple allelic inheritance. However, each individual will only inherit two of the three possible alleles. 

• Recall that the dominant version of a trait can mask the recessive version of a trait.  

  • Individuals with blood types A or B can have the homozygous dominant or heterozygous genotype.

• Individuals with blood type O will have the homozygous recessive genotype.

• Individuals with blood type AB have BOTH dominant alleles.  Since BOTH versions are equally expressed, we can say that this trait is Codominant. 

• A Punnett Square can be used to predict the blood type of offspring when the genotypes of the parents are known. 

  • Example: Two parents, one with blood type AB and the other with blood type O have a child.  They are concerned that the child has type B blood rather than a blood type of one of the parents.  Can a Punnett square be used to explain this? 

  • The Punnett Square shows the possible results of a “cross” between these two individuals.

  • The Punnett Square reveals that there is a 50% chance that a child of this couple will have Type A blood and a 50% chance of Type B blood.  There is a 0% chance of Blood type AB or O.

Color-blindness is another sex-linked disorder.  Predict what is happening in the cells that detect color in someone that is color-blind. 

Sex-Linkage

• Human cells contain 46 chromosomes.  Since chromosomes occur in pairs, this means that human cells have 23 pairs of chromosomes. 

  • Each chromosome pair contains genes for different traits.  Remember that one allele of the gene is located on the maternal chromosome while the other allele of the gene is located on the paternal chromosome. 

• A karyotype can be used to visualize the chromosome pairs.

  • The first 22 pairs are referred to as autosomes.

  • The 23rd pair are referred to as sex chromosomes.

    • Sex chromosomes determine the biological sex of an individual.  Those with 2 chromosomes at the X position (XX) are biological females.  Those with one chromosome at the X position and one at the Y position (XY) are biological males.

             Image from BSCS

• Genes for some human traits are found on the sex chromosomes. These are called sex-linked traits. While there are genes on the Y chromosome, we are specifically interested in genes that are found on the X chromosome.

  • Since females have two X chromosomes and males have only one X chromosome, the genotypes for these traits must be represented differently. 

• Hemophilia is a sex-linked disorder.  Individuals with hemophilia do not produce enough proteins that act as blood clotting factors. Normal protein production is the dominant trait while the mutated version is the recessive trait.

  • Alleles for a sex-linked trait are shown as being “carried” on the X chromosome.  Females will have two alleles for a sex-linked trait while males will only have one allele. 

• Example: A couple with normal blood clotting ability has a son who is diagnosed with hemophilia.  There is no family history of hemophilia on the father’s side of the family, but the mother’s great-grandfather did have the disorder.

• The Punnett Square shows the possible results of a “cross” between these two individuals.

• The Punnett Square reveals that the mother is a “carrier” for this disorder. There is a 50% chance that a son born to this couple will inherit hemophilia.

• Since males only have one allele for sex-linked traits, there is a higher probability of those traits being expressed in males. 

TOPIC: HOW CAN THE INHERITANCE OF A TRAIT IN A FAMILY BE SHOWN? (LS.Bio.7.1)

CMS BSCS Alignment: Lesson 10-13

NOTES

ESSENTIAL CONTENT 

• A pedigree is a chart that shows the inheritance of a trait in a family.

• Symbols are used to represent individuals on a pedigree. 

Assume the pedigree shown is for the inheritance of Cystic Fibrosis.  Label each individual on the pedigree with the correct genotype.

• A pedigree can be interpreted to determine the pattern of inheritance of a trait and the genotypes of the individuals.

• The pedigree shown below has three generations.  Neither of the parents of the siblings in generation three has the trait, yet it appears in one of their children.  This is a clue that this trait is recessive.  Both of the parents with a normal phenotype must be heterozygous carriers to have passed the trait to their child. 

TOPIC: HOW ARE TRAITS PASSED FROM PARENT TO OFFSPRING? 

(LS.Bio.1.5, LS.Bio.6.1, LS.Bio.6.2)

CMS BSCS Alignment: Lesson 14-15

NOTES

ESSENTIAL CONTENT 

Draw a simple picture of sperm and egg combining in fertilization. 

Sexual Reproduction

• Sexual reproduction is reproduction involving two sources of genetic material from two parents. This means that offspring will be genetic combinations of the two parents.

• A parent’s genetic material is passed to offspring by gametes. 

  • Gametes are sex cells.  The male gamete is sperm and the female gamete is egg. 

  • During sexual reproduction, the male and female gametes combine the genetic material of two parents in the process of fertilization.

• A fertilized egg is called a zygote.  The single-celled zygote will grow and develop into a multicellular organism through the process of cell division and cell differentiation.  

Write in the correct sequence of nucleotides that will be added to each side of the DNA molecule:

G -                       - C

T -                       - A

T -                       - A

G -                       - C

A -                       - T

DNA Replication

• Before a gamete is produced from a cell in the male or female sex organs, the DNA in that cell must be copied.  When the DNA is copied, the cell will have doubled chromosomes made of two sister chromatids attached at a centromere.

• Human body cells have 46 chromosomes made of DNA, each of which will be doubled before the process of cell division to produce the gametes begins.

• During DNA Replication, the hydrogen bonds connecting the two strands of DNA nucleotides are broken by a protein called an enzyme. This allows the two strands of nucleotides to separate. 

• Each strand of nucleotides is used as a template to create a new strand of nucleotides.  Free nucleotides are bonded to the open nucleotides according to base pairing rules until the entire molecule has been copied. Enzymes are involved in bonding all of the nucleotides on the new strand of DNA to each other and to the original strand. 

• At the end of DNA replication, two identical DNA molecules are complete.  These two identical DNA molecules form the sister chromatids of a doubled chromosome.

• Sometimes the DNA is copied incorrectly during this process.  This means that the gene is changed.  When the gene is changed, the protein that is coded for by that gene can also be changed.  

Label the diagram of Meiosis with the correct number of chromosomes for a human cell: 

Draw another simple diagram of fertilization, this time showing the number of chromosomes in the gametes and the zygote. 

Label each cell on your drawing as haploid or diploid. 

Meiosis

• Gametes are produced in the process of meiosis.  After the DNA has been replicated, the process of meiosis can begin.

• Recall that there are two copies of each type of chromosome, one from the individual’s mother and the other from the individual’s father.  These two copies make up a homologous pair.

  • In a human body cell, there are 23 homologous pairs of chromosomes. 

  • A cell with two of each type of chromosome is said to be diploid. 

• Process of Meiosis I:

  • Doubled chromosomes line up in the center of the cell in homologous pairs.

  • The homologous pairs are separated and pulled to opposite ends of the cell.

  • The cell divides down the middle.  At the end of Meiosis I, there are two daughter cells with HALF the original number of chromosomes.  The chromosomes in each daughter cell are still doubled since DNA was replicated before the process. 

• Process of Meiosis II: 

  • The chromosomes in each daughter cell line up down the middle of the cells.  They are no longer in homologous pairs. 

  • The sister chromatids that make up a doubled chromosome are pulled to opposite ends of the cells. 

  • The cells divide down the middle. At the end of Meiosis II there are four daughter cells from the original parent cell, each with HALF the original number of chromosomes.  The chromosomes are now singled.

• In order for sexual reproduction to occur, meiosis is necessary to produce gametes with HALF the normal number of chromosomes.

  • During fertilization, the male and female gametes combine in the process of fertilization to produce a zygote.  

  • That zygote should have one of each type of chromosome from the male parent (sperm) and one of each type of chromosome from the female parent (egg).

  • Therefore, the sperm and the egg should have HALF the normal number of chromosomes.  A cell with half the normal number of chromosomes (one of each type) is said to be haploid. 

  • Two haploid gametes combine in fertilization to make a diploid zygote. 

Nondisjunction

• Sometimes, during meiosis, the chromosomes do not separate correctly. When this happens it is called nondisjunction. This can result in a gamete with an incorrect number of chromosomes. 

• At the end of meiosis in human cells, there should be 4 haploid cells, each with 23 chromosomes.  However, if the chromosomes do not separate correctly during the process, one of those cells may end up with one extra chromosome while another ends up with one missing chromosome. 

• Gametes that have an incorrect number of chromosomes can result in a zygote with an incorrect number of chromosomes after fertilization takes place.

• Nondisjunction results in chromosomal conditions such as Down Syndrome.  People with Down Syndrome have an extra copy of Chromosome 21.  This is called trisomy 21.

  • Because each chromosome has multiple genes, all of those genes are affected by a chromosomal condition.

  • The cause of chromosomal conditions can be seen on a karyotype, or a picture of the chromosomes.  Mutations that cause gene disorders cannot be seen on a karyotype since mutations occur at a molecular level.

TOPIC: WHY ARE SIBLINGS FROM THE SAME PARENT DIFFERENT? (LS.Bio.6.2)

CMS BSCS Alignment: Lesson 16

NOTES

ESSENTIAL CONTENT 

Name a source of genetic variation that has already been discussed. 

Genetic Variation

• Genetic variation is a term that describes the differences in DNA among individuals.

• All humans have DNA that codes for human traits, but there are differences in the expression of those traits in all individuals.

• Genetic variation allows for some individuals to have favorable traits that provide a reproductive advantage.  

• In a changing environment, these favorable variations will be selected and passed to offspring.  This process of natural selection is what allows populations to evolve adaptations.  

• Meiosis is one source of genetic variation.

The diagram below shows maternal chromosomes and paternal chromosomes in different colors.  In the circles provided, show 4 different ways these chromosomes can be sorted during meiosis. 

Independent Assortment

• Remember that a homologous pair consists of one maternal and one paternal chromosome of a particular type, carrying alleles for genes of the same traits.

• When homologous pairs of chromosomes line up at the middle of the cell during meiosis I, they can line up in any order every time meiosis occurs.  In other words, all of the chromosomes inherited from the mother don’t line up on one side and all of the chromosomes inherited from the father on the other side. 

• Since there are 23 pairs of human chromosomes, there are over 8 million possible ways that those chromosome pairs could line up and be sorted into new cells. 

• This is one reason that every gamete in an individual is slightly different from the other gametes in that same individual. 

How can crossing over make it harder to predict the inheritance of a polygenic trait such as high cholesterol? 

Crossing Over

• All of the genes located on one chromosome are linked together and can be inherited together.

• However, when chromosomes line up in meiosis as homologous pairs, small pieces of the chromosome can switch places.  This is called crossing over.

• Since alleles for a gene may be different on the maternal and paternal chromosome, the process of crossing over can change the combination of alleles that will be linked.

• This provides another source of genetic variation as a result of meiosis.