Chapter 12 Inheritance Patterns and Human Genetics

MORGAN In early 1900’s, geneticist Thomas Hunt Morgan begin breeding Drosophila, the fruit fly -Drosophila has four pairs of chromosomes -Morgan observed that one chromosome was different in males than in females -He called this smaller chromosome the X chromosome, and the larger chromosome the Y chromosome

Morgan hypothesized that the X and Y chromosomes are sex chromosomes -All other chromosomes are autosomes Like other chromosomes, sex chromosomes form pairs and segregate into separate cells during meiosis I - Gametes that form in meiosis II have either an X chromosome or a Y chromosome

- Gametes produced by males can contain either an X chromosome or a Y chromosome -Gametes produced by females contain only X chromosomes MALES DETERMINE SEX OF AN OFFSPRING

In humans and fruit flies, an egg that is fertilized by a sperm with an X chromosome will be a female zygote (XX) and an egg that is fertilized with a Y chromosome will be a male zygote (XY) - This system means that 50% of offspring of any mating will be male, and 50% will be female

Sex Linkage  X-linked genes- genes found on the X chromosome  Y-linked genes- genes found on the Y- chromosome  The presence of a gene on a sex chromosome is called Sex Linkage

 Morgan experimented with eye color in Drosophila

Although most fruit flies have red eyes, a few males have white eyes -When Morgan crossed a white eyed male with a red-eyed female: F1= all red eyed flies F2= 3 red :1 white BUT ALL WHITE-EYED FLIES WERE MALE! WHY NO FEMALES?

Morgan hypothesized that the gene for eye color must be carried on the X chromosome -Eye color in Drosophila is an X-linked trait -Another cross:

Linkage Groups  There are thousands of genes, so there must be more than one per chromosome  Linkage Group- genes located on one chromosome  Another experiment!

MORE FLIES! - In Drosophila, the allele G for gray body is dominant over the allele g for black body -The allele L for long wings is dominant over l for short wings Morgan crossed homozygous gray, long- winged flies (GGLL) with homozygous black, short-winged flies (ggll) to produce a heterozygous F1 generation: GgLl

He then crossed members of this F1 generation (GgLl x GgLl) to produce an F2 generation -Morgan knew that if alleles for body color and wing length were on different chromosomes, they should assort independently and produce an F2 generation with a phenotypic ratio of 9:3:3:1

-Morgan predicted that if alleles were on the same chromosome, he would get a 3:1 ratio in the F2 generation -His results were a 3:1 phenotypic ratio (3 gray long : 1 black short) -Morgan hypothesized that the genes for body color and wing length are LINKED

Crossing-Over Though the 3:1 ratio was produced, there were still a few offspring that had the genotype Ggll and ggLl HOW??? - Crossing over caused the linked traits to be inherited independently and not together Do you remember what happens in crossing- over?

CROSSING OVER- the exchange of pieces of DNA between homologous chromosomes - This accounts for the unexpected phenotypes that Morgan obtained

Chromosome Mapping The farther apart 2 genes are on a chromosome, the greater the chance of crossing-over occurring  Chromosome Map- diagram that shows the linear sequence of genes on a chromosome

Mutation  Remember, a mutation is the change in the DNA of an organism  Can be helpful or harmful - Some mutations help an organism adapt to new surroundings - Some mutations cause diseases (cancer)

Different types of Mutations 1. Germ-cell: occur in gametes, do not affect organism 2. Somatic: occur in body cells, can affect organism Ex: Skin cancer and Leukemia 3. Lethal Mutations- cause death, usually before birth

Chromosome Mutations 1. Deletion- loss of a piece of chromosome due to chromosomal breakage 2. Inversion- segment breaks off and then reattaches in reverse to the same chromosome

Chromosome Mutations cont. 3. Translocation- chromosome breaks off and then reattaches to another homologous chromosome 4. Nondisjunction- failure of a chromosome to separate from its homologue during meiosis - one gamete gets an extra chromosome, and one is missing one

Gene Mutations Point Mutations- substitution, addition, or removal of a single nucleotide 1. Substitutions- one nucleotide in a codon is replaced with a different nucleotide, resulting in a new codon Sickle-cell anemia- adenine is substituted for thymine, causes misshaped red blood cells

2. Frame-shift mutations- addition or deletion of a nucleotide can cause other codons to shift

12-2 Human Genetics  Much of what has been discovered about the inheritance of traits has been learned in the study of human genetics

Studying Human Inheritance  Humans have up to 20 times as many genes as the fruit fly, so inheritance is more complicated  Pedigrees –Family records that show how a trait is inherited over several generations –Certain phenotypes are usually repeated in predicable patterns- called Patterns of Inheritance

 Carriers- individuals who have one copy of a recessive autosomal allele –Do not express the allele, but can pass it to their offspring

Genetic Traits and Disorders Genetic Disorders- diseases or debilitating conditions that have a genetic basis 1. Single-allele traits 2. Multiple-allele traits 3. Polygenic traits 4. X-linked traits 5. Sex-influenced traits

Single-allele traits Traits controlled by a single allele of a gene Huntington’s Disease- caused by a dominant allele located on an autosome - mild forgetfulness, irritability, in 30’s or 40’s - in time causes loss of muscle control, spasms, severe mental illness and death - most people don’t know they have the disease until they have passed it onto their children

Huntington's pedigree Huntington's pedigree

Genetic Marker- a short section of DNA that is known to have a close association with a particular gene located nearby - can give clues that genes such as the one that causes HD is present Other Diseases: - Sickle-cell anemia and Cystic Fybrosis

Multiple-allele traits Controlled by 3 or more alleles of the same gene that code for a single trait –In humans, the ABO blood groups are controlled by 3 alleles, I A, I B, and i –Each individual’s blood type consists of two of these alleles

I A I A genotype- type A blood I A i genotype- type A blood I B I B genotype- type B blood I B i genotype- type B blood ii genotype- type O blood I A I B genotype- type AB blood

Polygenic Traits A trait controlled by 2 or more genes Ex: Skin color is controlled by 3 to 6 genes - Each gene results in a certain amount of melanin- pigment found in skin - Each gene results in a certain amount of melanin- pigment found in skin Ex: Eye color Many genes are also controlled by environment - Height is influenced by nutrition and disease

X-Linked Traits Colorblindness- recessive X-linked disorder in which an individual cannot distinguish between certain colors Hemophilia- disease in which the ability of the blood to clot is impaired Duchene Muscular Dystrophy- weakens and destroys muscle tissue NOT ALL X-LINKED TRAITS ARE DISEASES

Sex-influenced traits Needs to be a presence of male or female sex hormones in order for the trait to be expressed –Ex: Baldness- allele that causes baldness in men does not in women

Disorders due to Nondisjunction Causes gamete to either lack a chromosome or have an extra one Normal chromosome # Monosomy- zygote has only one copy of a particular chromosome (45 chromosomes) 2. Trisomy- zygote has 3 copies of a particular chromosome (47 chromosomes)

Some abnormalities in chromosome number can be lethal, others are not -Down Syndrome- extra chromosome, causes mild to severe mental retardation Nondisjunction can also affect the sex chromosomes -Klinefelter’s Syndrome- males with an extra X chromosome (XXY) * these individuals have some feminine characteristics and are sometimes mentally retarded and infertile

-Turner’s Syndrome- individuals with a single X chromosome (XO) * These individuals have a female appearance but do not mature sexually and remain infertile - Individuals with a single Y chromosome do not survive

Detecting Genetic Disorders Genetic Screening- an examination of a person’s genetic makeup - May involve constructing a karyotype Genetic Counseling- a form of medical guidance that informs people about problems that could affect their offspring

Diagnosing 1.Amniocentesis- a small amount of fluid from the sac that surrounds the fetus is removed and analyzed - between 14 th and 16 th week of pregnancy - fetal cells and proteins from fluid can be analyzed and a karyotype can be prepared 2. Chorionic villi sampling- a sample of the tissue chorionic villi is taken and analyzed - 8 th and 10 th week of pregnancy

Sometimes tests are performed immediately after birth - Phenylketonuria- genetic disorder in which the body cannot metabolize the amino acid phenylalanine - the accumulation of this amino acid can cause severe brain damage - can be easily controlled by diet if known