HumanGeneticsPart 3

11.3 HUMAN GENETICS • Linkage:

Genes that are on the same chromosome are said to be LINKED ,

• and the general phenomenon in which genes occur on the same chromosome is called LINKAGE.

• Genes may be linked other than just on the sex chromosomes. Linked genes do not assort INDEPENDENTLY, and do not result in the predicted Mendelian ratios for two or more genes .

• Linkage was first observed by WILLIAM BATESON and RC PUNNETT in the early 1900s in pea plants.

• Bateson and Punnett crossed true breeding plants with purple flowers and long pollen grains (genotype PPLL) to red flowers and round pollen grains ( ppll).

• The F1 plants (PpLl) were then crossed among each other.

• Mendel’s law would predict a 9:3:3:1 ratio for an independently assorting, dihybrid cross, The F2 generation did not adhere to this ratio:

• Batson and Punnett noticed that – two types of F2 peas ( P_L_ ) and (ppll) were more

abundant – and the other two types of peas ( P_ll ) and (ppL_)

were less abundant.

• Bateson and Punnett hypothesized that the more prevalent allele combinations could reflect genes that are transmitted on the same chromosome and do not separate during meiosis.

Crossing-Over:

• Thomas H. Morgan noticed that sometimes not all alleles in a linkage group were inherited together.

• When he crossed pairs of flies he did not get the expected results.

• He could explained that his outcomes were a result of crossing over- the exchange of alleles between homologous chromosomes.

LINKAGE MAPS

• The correlation between crossover frequency and the distance between genes is used to construct LINKAGE MAPS (Chromosome Maps).

• Crossing-over occurs more often between some alleles than between others. – Since most alleles occupy a fixed place on a

chromosome.

– The smaller the distance between two alleles, the greater the likelihood that these alleles will not be separated by crossing over.

– Data from crossing over was used by Alfred Sturtevant to construct a LINKAGE MAP(chromosome map).

– This was a diagram of allele positions on a particular chromosome.

• To construct a chromosome map researchers compare FREQUENCY of crossover for three characteristics such as eye color, body color and wing shape.

• Two alleles that are separated by crossing over 1 percent of the time is only 1MAP UNIT apart. – That means that two

alleles that are separated by crossing over 10 percent of the time is 10 MAP UNITS apart.

• Scientist cannot use the same techniques used to study organisms such as Drosophila to study human inheritance.

• There are three tools that geneticists use in studies human genetics.

Studying Humans

• 1. Population Sampling: the technique of selecting a small number of individuals to represent larger population.

• 2.Twin Studies: The study of twins (especially twins raise in different environments) to distinguish between genetic and environmental influences.

• What is the difference between identical and fraternal twins? How do Siamese or conjoined twins occur? (FYI, not in notes!)

• 3. Pedigree Studies: Geneticists use diagrams to trace a family record.

– These diagrams often reveals a CARRIER, someone who is heterozygous for a trait.

– These types of flow charts reveal those individuals that are carriers.

– Families are the primary tool of the human geneticist and the larger the family the better.

– The more children in a generation , the easier it is to discern patterns or modes of inheritance.

– Family relationships and phenotypes are displayed in a standard chart called a PEDIGREE. A pedigree is built of shapes connected by lines.

• In 1845 a genealogist named Pliny Earle constructed a pedigree of a family with colorblindness, using MUSICAL NOTATION-– half notes for unaffected females,– quarter notes for colorblind females,– and filled-in and squared-off notes to

represent the many colorblind males.

First Pedigree

A Historical Pedigree:

• This pedigree is of Egypt’s Ptolemy dynasty and shows only genealogy, not traits.

• It appears almost ladderlike

• because of the extensive inbreeding. • From 323 B.C. to Cleopatra’s death in 30

B.C. , the family experienced five brother-sister pairings, plus an uncle-niece relationship.

• Cleopatra married her brother, Ptolemy XIII, when he was ten years old. These type of marriages were an attempt to preserve the royal blood.

How do we use pedigrees today?:

• Pedigrees are used to help families identify the risk of transmitting an inherited illness and as starting points for gene searches.

• Researchers looked for the gene that causes Huntington disease took researchers to a remote village in Venezuela and a large family pedigree.

• The gene was eventually traced to a Portuguese sailor believed to have introduced the mutant gene in a fishing village.

Huntington’s

• Through genetic studies geneticists have identified several patterns of inheritance.* Single Allele Traits: traits controlled by a single allele, i.e., Sickle Cell Anemia.

* Polygenic Traits: traits that controlled by two or more genes. – Example… skin color in which four to seven genes

control this trait. Each gene has an additive effect.

• * Multiple Allele Traits: some traits are controlled by three or more alleles. One example is human blood type.

• IA , IB , IO , are alleles that code for blood type. – The alleles IA and IB are co-dominant and both are

dominant to IO.

• * Sex linked (X-linked) traits: Human traits that appear only on the X chromosome.

• Males have only one X chromosome, any recessive allele on the X chromosome will be expressed because it cannot be masked by a dominant allele.

• One example is color blindness.

*Another sex linked trait is Hemophilia

• Nondisjunction: A disorder in which there is a failure of chromatids to separate during cell division.

• When this occurs the sperm or the egg will receive an extra copy or a missing chromosome.

• If a zygote has a condition in which there are three of a particular chromosome, the condition is called trisomy.

•

• Two examples are Down’s Syndrome and Klinefelter’s Syndrome.(XXY, jot this down!)

Detecting Genetic Disorders• When there is a history of genetic disorders an

individual may desire to undergo genetic screening.

• This is a process of examining a persons genetic makeup.

Karyotyping

A karyotype is one method of genetic screening. This is a process of taking a picture of an individual’s chromosomes.

• Another tool of genetic screening is amniocentesis.

• This is the removal of a small amount of amniotic fluid from the amnion, the sac that surround the fetus.

• Physicians use the fluid to make karyotypes.

• Another method of sampling fetal chromosomes is through chorion villus sampling. The chorion villa grows between the mother’s uterus and the placenta and has the same genetic makeup as the fetus.