Unit 1 of Genetics

Preformationism was introduced during the late 17th-late 18th century where it was believed that offspring arise as a result of what?

an unfolding and growth of a preformed individual, which exists within the egg or the sperm

During the time of Preformationism, what did ovists believe?

eggs contained performed individual

During the time of Preformationism, what did spermists believe?

preformed human contained within the head of the sperm

what is epigenesis?

belief that organs and structure of the adult arose during the course of the embryonic development

Charles Darwin introduced the idea of inheritance of acquired characteristics. How did he make sense of this concept?

He believed that the body shed gemmules, which collected in the reproductive organs prior to fertilization. These gemmules contained atavisms, traits that were present in the organism’s ancestors, which laid dormant until “reawakened”

What did the Theory of Pangenesis suggest?

That gemmules were controlling traits that were being passed down from ancestor to offspring. The theory was tested by Francis Galton. He suggest that if gemmules are present in blood, then the donor should pass down he/her traits to the recipient. He had disproven the theory.

What is a genotype?

the genetic makeup (constitution) of an organism

ex. TT, Tt, tt

What is a phenotype?

The expression of the genotype as an observable physical, biochemical, or physiological trait

ex. tall or short plant height

What does the term homozygous mean?

possessing two copies of the SAME allele of a gene

ex. TT or tt

What does the term heterozygous mean?

possessing two DIFFERENT alleles of a particular gene

ex. Tt or Pp

What does the term hemizygous mean?

possessing only a single allele of a particular gene because the individual possesses only one copy of the chromosome on which that gene is located

ex. human males are hemizygous for genes located on their X chromosome and also for those located on their Y chromosome

What are somatic cells?

Any cell other than a reproductive cell

ex. skin, blood, bone, muscle, nerve, etc.

What are gametes?

reproductive cell

ex. sperm, egg

What is a zygote?

cell formed by the fusion of a sperm and egg

What does the term haploid mean?

cell or organism containing ONE set of chromosomes in each of its cells (n)

ex. haploid gametes in humans contain. 23 chromosomes

What does the term diploid mean?

cell or organism containing TWO sets of chromosomes in each of its cells (2n)

ex. diploid cells in humans contain 46 chromosomes (2n=46)

What are autosomal chromosomes?

all of the chromosomes except the sex chromosomes, which males and females possess two copies of each

ex. chromosomes #1-22 in humans

What are sex chromosomes?

chromosomes that differ in male and females

ex. X and Y chromosomes in humans with males having XY and females having XX

What are dominant alleles?

• expressed when homozygous or heterozygous

• can “hide” the presence of the recessive allele

• usually represented by a capital letter

ex. F= freckles so two possible genotypes would be FF and Ff

What are recessive alleles?

• expressed when homozygous or hemizygous

• are “hidden in the presence of the dominant allele”

• usually represented by lower came letter

ex. c = cystic fibrosis allele, so one possible genotype would be cc

What happens during Prophase of Mitosis?

• chromosomes condense

• spindle apparatus is assembled

• nuclear membrane disintegrates

What happens during Metaphase of Mitosis?

chromosomes line up individually along the equator of spindle apparatus

What happens during Anaphase of Mitosis?

• centromeres divide

• daughter chromosomes begin to move to opposite poles

What happens during Telophase of Mitosis?

• daughter chromosomes finish moving to opposite poles

• chromosome decondense

• nuclear membrane reforms

• spindle apparatus disassembles

What is cytokinesis in Mitosis?

cell division resulting in two daughter cells containing the same types and numbers of chromosomes as original parent cell

What happens during Prophase I of Meiosis I?

• chromosomes condense

• homologous chromosomes undergo pairing

• spindle apparatus assembles

• nuclear membrane disintegrates

What happens during Metaphase I of Meiosis I?

chromosomes line up in pairs along the equator of the spindle apparatus

What happens during Anaphase I of Meiosis I?

• members of each pair of homologous chromosomes begin to move to opposite poles

• centromeres do not divide

What happens during Telophase I of Meiosis I?

• members of each pair of homologous chromosomes finish moving to opposite poles

• spindle apparatus disassembles

• nuclear membranes reform

What happens during cytokinesis of Meiosis I?

cell division occurs resulting in two haploid daughter cells, each one containing ONE copy of each chromosome

What happens during Prophase II of Meiosis II?

• chromosomes condense

• spindle apparatus reassembles

• nuclear membrane disintegrates

What happens during Metaphase II of Meiosis II?

chromosomes lines up individually along equator of spindle apparatus

What happens during Anaphase II of Meiosis II?

centromeres divide and sister chromatids begin moving to opposite poles

What happens during Telophase II of Meiosis II?

• daughter chromosome finish moving to opposite poles

• spindle apparatus disassembles

• nuclear membrane reforms

What happens during cytokinesis of Meiosis II?

cell division; produces four haploid daughter cells and each contains ONE copy of each gene and chromosome

What happens during spermatogenesis?

spermatogonial cells undergo another mitotic division to produce primary spermatocytes, then undergo Meiosis I to produce secondary spermatocytes, then undergo Meiosis II to produce 4 spermatids then go through maturation to get 4 sperm in the end

What are important features of spermatogenesis?

• requires 48 hours from beginning of meiosis until functional sperm is produced to fertilize an egg

• 64 days total required from time when spermatogonial cells divide (mitosis) to primary spermatocytes become mature sperm

• 200-300 million cells are released at one time

What happens during oogenesis?

• oogonial cells in ovary undergo one final mitotic division (primary oocytes)

• 2-3 months after conception, primary oocytes begin Meiosis I, but meiosis is arrested in Prophase I

• cells remain arrested until puberty

• beginning at puberty, one cell per month resumes meiosis

• 1 primary oocyte produces one secondary oocyte

• Meiosis II arrested in Metaphase II until fertilization

• After fertilization, Meiosis II is complete, producing one ovum and 3 polar bodies

What are important features of oogenesis?

• can require 12-50+ years to produce and ovum

• oogenesis ends at menopause

• 400-450 eggs are produced in a woman’s lifetime

What are sister chromatids?

Comprising of a replicated chromosomes containing the same alleles of a gene

What are daughter chromosomes?

Sister chromatids after they separate during mitosis or meiosis

How many chromatids do unreplicated chromosomes have?

one

How many chromatids do replicated chromosomes have?

two, joined at the centromere

What are homologous chromosomes?

two members of a specific pair of chromosomes having the same length, same centromere positioning, same banding pattern when stained, and contain same gene arranged in the same linear order

What are alleles?

alternative versions of the same gene

ex. T= tall plant; t= short plant

What does Mendel’s Law of Segregation suggest?

during gamete formation, the two members of each gene pair segregate (separate) from each other during gamete formation, and one-half of the gametes will carry one member of the gene pair, and the other half will carry the other members of the gene pair

What does Mendel’s Law of Independent Assortment suggest?

during gamete formation, the segregation of one gene pair is independent of the segregation of other gene pairs

What does it mean for a genetic trait to be autosomal dominant?

allele is responsible for the trait is dominant, and the gene is physically located on a specific autosomal chromosome

What does it mean for a genetic trait to be autosomal recessive?

allele is responsible for the trait is recessive, and the gene is physically located on a specific autosomal chromosome

What does it mean for a genetic trait to be X-linked dominant?

allele is responsible for the trait is dominant, and the gene is physically located on the X chromosome

What does it mean for a genetic trait to be X-linked recessive?

allele is responsible for the trait is recessive, and the gene is physically located on the X chromosome

What does it mean for a genetic trait to be Y-linked (holandric)?

allele is responsible for the trait is physically located on the Y chromosome

What is complete dominance?

situation in which the dominant allele completely “hides” the presence of the recessive allele, and heterozygous individuals express the dominant phenotype

ex. In a cross between SS x ss, the F1 offspring have the genotype Ss, and a smooth phenotype

What is incomplete dominance?

heterozygous individuals (Aa) exhibit a phenotype intermediate between those of either homozygote (AA or aa)

ex. one red rose and one white rose create an all pink rose

What is codominance?

both members of a gene pair are fully expressed in heterozygous individuals

ex. MN blood types

What does it mean to have multiple alleles?

situation in which more than two different alleles of a particular gene may exist in a population and an individual possesses some combination of two of these

ex. ABO blood group

What is pleiotropy?

one gene affects more than one, seemingly unrelated phenotypic characteristic or trait

ex. achondroplasia affects limbs, forehead, legs, spinal column, etc.

What is penetrance?

percentage of individuals with a given phenotype who exhibit the phenotype normally associated with that genotype

What is complete penetrance?

everyone having the appropriate genotype expresses the expected phenotype

What is reduced penetrance?

a certain percentage of individuals who inherit the appropriate genotype fail to exhibit the expected phenotype

What is expressivity?

a measure of the degree to which a given allele is expressed at the phenotypic level (measures intensity or severity)

What is variable expressitivity?

occurs when a phenotype is expressed to a different degree among individuals having the same genotype

What is euploid?

a cell or organism having any number of complete sets of chromosomes

ex. haploid or diploid

What is polyploid?

cells or organisms having more than two complete sets of chromosomes

ex. triploid, tetraploid, pentaploid, etc.

What is Aneuploid?

• involves the loss or gain of one or a few chromosomes, but not an entire set

• cell or organism whose chromosome number differs from the wild type by only part of a chromosome set

ex. monosomic (2n-1), trisomic (2n+1), nullisomic (2n-2), and double trisomic (2n+1+1)

What is non-disjunction?

failure of chromosomes to separate properly during meiosis I or meiosis II which can result in gametes containing one additional chromosome or one less chromosome than normal

→ can give rise to trisomic (2n+1) or monosomic (2n-1)

What is X-inactivation?

when the X chromosome is inactive and observe “patchy” expression of X-linked genes in female mammals

What are barr bodies?

the number of X chromosomes present minus 1

What are the phenotypic traits of Monosomy X (Turner Syndrome)?

sterile female, short stature (under 4’8”), streak ovaries, no menstrual cycle, failure to develop characteristics of puberty, typical intelligence, possible learning disabilities, possible narrow aorta, possible heart murmur, webbed neck, and puffy hands and feet at birth

What are phenotypic traits of Klinefelter Syndrome (XXY)?

male, tall, slender, failure to obtain characteristics of puberty (little beard growth, sparse body hair, voice may not deepen, narrow waist, broader hips, and some breast development), fertility problems (small testes and low testosterone), and typical IQ but language and reading difficulties

What are phenotypic traits of XXY Syndrome?

taller than average

What are phenotypic traits of Trisomy X?

tend to be taller than average, normal fertility, typical IQ, and may have learning diabilities

What are phenotypic traits of Down Syndrome (Trisomy 21)?

intellectual impairment, heart defects, epicanthal fold in inner corner of eye, slower than average growth, wide & flat skull, large tongue, increased risk of developing leukemia, and increased risk of early onset Alzheimer’s disease

What are phenotypic traits of Edwards Syndrome (Trisomy 18)?

low birth weight, severe intellectual impairment, severe growth delays, severe developmental delays, major heart defect, abnormalities of kidneys & intestines, esophagus does not connect to stomach, clenching of hands, rocker bottom feet, and low-set ears

What are phenotypic traits of Patau Syndrome (Trisomy 13)?

severe abnormalities of many organ systems, severe intellectual impairment, severe heart defects, severe abnormalities of brain, small head, facial malformations (cleft lip, cleft palate, small eyes or missing, may only have a single eye), polydactyly, seizures, and skeletal abnormalities

What are common reasons for performing prenatal testing?

• mother is over the age of 35

• previous child with genetic defect

• family history of a genetic condition

• parent(s) belong to a group at high risk of a particular genetic condition

• if the individual or her spouse/partner are known to be carriers

• one parent has structural chromosomal abnormality

• exposure to certain hazardous agents

• family history of X-linked disorder

How do ultrasounds work?

use sound waves moving through the amniotic fluid to visualize the fetus

How does amniocentesis work?

take sample of amniotic fluid surrounding the developing fetus by inserting needle through wall of abdomen, into uterus, and withdraw amniotic fluid

How does karyotyping work?

a cell sample is obtained and grown in culture

• chemicals are added to the cells growing in the culture to stop mitosis in metaphase, when the chromosomes are visible

• chromosomes stained and photographed, then arranged in pairs on karyotype sheet

• chromosomes are examined to detect changes in chromosome number or structure

• computerized methods are commonly used

How does Chorionic Villus Sampling (CVS) work?

use ultrasound to locate fetus, and fetal cells are removed using a catheter inserted through the cervix or through abdominal wall

When are ultrasounds used?

very early pregnancy through time of delivery

Why are ultrasounds used?

• determine age

• detect multiple pregnancies

• detect congenital abnormalities

• used in conjunction with amniocentesis and CVS

When is karyotyping performed?

• in conjunction with amniocentesis: 15th-18th week of pregnancy

• in conjunction with CVS: 10th-12th week of pregnancy

What does amniocentesis test for?

• test fluid for alpha fetoprotein (for neural tube defects) and biochemical abnormalities

• test cells for chromosomal abnormalities, DNA testing, and immunological tests

What are the risks of amniocentesis and CVS?

miscarriage, injury to fetus or mother, infection to mother or fetus, and Rh sensitization

What are the advantages of CVS?

can be performed early in pregnancy and cells are actively dividing so karyotyping can be performed without growing cells in culture

What are restriction enzymes?

recognize specific base sequences within a DNA molecules and cut both strands of the DNA molecule within that base sequence

What are RLFPs?

differences in the specific patterns of restriction fragments that are observed when the DNA from different individuals is cut with the same restriction enzyme

How do we detect RFLPs?

obtain DNA sample, cut DNA with restriction enzyme, producing DNA fragments of different sizes, and the DNA fragments are separated on the basis of their size and electrical charge using gel electrophoresis

How are RFLPs used to diagnose genetic disorders and detect carriers of genetic disorders?

restriction enzymes recognize specific base sequences in the DNA so mutations and changes in the chromosome can alter the base sequence of the DNA molecule allowing the DNA from that individual to be cut into a different combination of restriction fragments by the enzyme

What is diagnostic testing?

involves identifying current disease states, which is included in pre-natal and newborn testing

What is carrier testing?

determines whether an individual is a carrier of a genetic trait, which is often performed on individuals in the population who may have no personal or family history of a condition, but have a greater than average chance of carrying a particular faulty gene due to ancestry

What is predictive testing?

used to determine whether a person possess a genetic mutation that will lead to a late-onset disorder

What is Phenylketonuria (PKU)?

• inability to convert amino acid phenylalanine to tyrosine due to mutation in gene coding for phenylalanine hydroxylase

• since phenylalanine hydroxylase is absent, phenylalanine is converted to phenylpyruvic acid (toxic)

• resulting in brain damage, intellectual disability, and seizures

What are phenotypic symptoms of Phenylketonuria (PKU)?

• normal at birth

• if undetected and untreated → irreversible mental impairment, seizures, microcephaly, behavioral/social problems, hyperactivity, stunted growth, skin rashes, fair skin, and blue eyes

Why was a genetic screening program made for Phenylketonuria (PKU)?

it is critical that pregnant women are on a low phenylalanine diet through pregnancy because high levels will cause severe brain damage to fetus, abnormally small head, heart defects, and low birth weight

What are the phenotypic traits of Sickle Cell Anemia?

sickle shaped red blood cells, anemia, fatigue, shortness of breath, blood vessels blocked, organs become damaged, spleen becomes enlarged, and reproductive problems

What were the negative aspects of sickle cell screening programs?

lack of sensitivity to issues of race, misinterpretation of test results, stigmatization of sickle cell carriers, and inadequate protection of patients rights’

What are symptoms of Tay-Sachs disease?

decreased eye contact & blindness, lose muscle tone & strength, mental & social skills delayed, increased startle response, lost motor skills, dementia, paralysis/loss of muscle function, seizures, slowed growth, and loss of ability to swallow

Why was a genetic screening program made for Tay-Sachs disease?

allow couples to have children who do not have the disease