Module 10 - Human Genetics

Question 1

What is a character? What is a trait?

Answer 1

character: a heritable, observable feature such as height, hair colour, eye colour
trait: a variant of a character, e.g brown eyes or blue eyes, red hair or brown hair

Question 2

How are traits transmitted to offspring?

Answer 2

• the information (instructions) that determines characters is coded as DNA, stored in the nucleus
• the sequence of DNA that determines a particular character trait, e.g eye colour is called a GENE
• genes encode proteins that determine both the structure and function of a cell or oganism
• genes exist at specific locations (like a postal address) on a chromosome
• chromosomes are highly coiled and packaged strands of DNA, humans have 46 chromosomes

Question 3

What is the language of DNA?

Answer 3

• DNA = deoxyribonucleic acid
• made of a polymer of nucleotides. one strand = nucleic acid
• nucleotide basis are Adenine, Guanine, Cytosine, Thymine
• exists mainly as a double strand of hydrogen-bonded base pairs - ALWAYS A-T and G-C
• form triplet words or codons that encode into 20 amino acids, known as the genetic code

remember: proteins are chains of amino acids

Question 4

How does genetic information flow?

Answer 4

transcription: in the nucleus, the synthesis of a messenger RNA (mRNA) molecule from the DNA template
translation: mRNA is decoded by a ribosome, outside the nucleus to produce a specific amino acid chain, e.g., a polypeptide or protein

Question 5

What is gene expression?

Answer 5

gene expression is the process by which information from a gene is used in the synthesis of a functional gene product e.g a protein

Question 6

What are chromosomes?

Answer 6

• DNA is packaged into condensed units in the cell nucleus called chromosomes – normally 46 in humans
• comprise DNA molecules and proteins – mostly histones – highly coiled into a chromatin fibre
• during cell division the DNA replicates producing 2 identical sister chromatids held together by a structure called the centromere

Question 7

How do human chromosomes work?

Answer 7

• humans have a total of 46 chromosomes
• there are 22 pairs of homologous, autosomal (non-sex) chromosomes
• homologous chromosomes: contain the same genes in the same order (but may be different gene versions)
• one of the pair is derived from the mother and the other is from the father
• there is also one pair of sex chromosomes
• females have two homologous X chromosomes = XX
• males have only one X and one Y chromosome = XY

Question 8

What is a karyotype?

Answer 8

chromosomes of an individual can be stained and sorted by size and shape for analysis - this is called a karyotype

Question 9

What is the function of mitosis? what is the function of meiosis?

Answer 9

mitosis: normal cell division for growth and repair
meiosis: reduction in half of the number of chromosomes during gametogenesis - the production of eggs or sperms

daughter cells of MITOSIS are DIPLOID CELLS = 2 sets of chromosomes

daughter cells of MEIOSIS are HAPLOID CELLS = one set of chromosomes

Question 10

What is a diploid vs haploid cell?

Answer 10

diploid cell (i.e body cells): two homologous sets of chromosomes

haploid cells (eggs, sperm = gametes): one set of chromosomes

Question 11

what is a gamete?

Answer 11

a mature haploid male or female germ cell (i.e sperm or egg) that is able to unite with another of the opposite sex in sexual reproduction to form a zygote (a single cell embryo) that then undergoes repeated mitosis and differentiation in development

Question 12

What are homologous chromosomes?

Answer 12

a pair of chromosomes that look the same and have the same sequence of genes, but may have different versions = alleles

Question 13

Define character, trait, gene, allele:

Answer 13

character: a heritable, observable feature such as hair colour, eye colour
trait: a variant of a character, e.g brown eyes or blue eyes, red hair or brown hair
gene: unit of heredity which controls a particular character e.g eye colour
allele: alternative form of a gene that codes for an inherited trait, located at the same position on homologous chromosomes

Question 14

What does it mean by alleles can be “dominant” or “recessive” ?

Answer 14

dominant alleles: dominates the other trait

• are always expressed
• mask the expression of recessive allele
• notation = capital letter “A”

recessive alleles: are dominated by the other trait

• expressed only when on both alleles (homozygous)
• presence is masked by dominant allele
• therefore recessive traits: may be hidden in an individual called the “carrier”
• notation = smaller letter “a”

Question 15

what is homozygous and what is heterozygous?

Answer 15

homozygous: same alleles on homologous chromosomes
- homozygous dominant AA
- homozygous recessive aa

heterozygous: different alleles on homologous chromosomes
- Aa
- “carrier” = a heterozygous individual who “carries” the recessive gene, since it is not expressed

Question 16

What is a genotype? what is a phenotype?

Answer 16

genotype: can be used in two ways:
- total inherited gene allele versions that exist in a particular human (or other organism)
- genotype of a trait: a gene or set of genes which is responsible for a particular character trait, i.e described by the dominant and recessive alleles

phenotype: can be used in two ways:
- the total observable physical traits of an individual
- in reference to one particular trait that is measurable and is expressed
- therefore the genotype determines the phenotype - whether at the character level or the individual organism level

Question 17

What are mutations? what are the types and causes?

Answer 17

changes in the nucleotide sequence of DNA 
Types of mutations: 
1. single gene mutations: 
-substitutions 
-insertions 
-deletion of nucleotides (bases) 

2. chromosomal mutations:
   - duplications

causes of mutations:
1. errors in DNA function: spontaneous mutations, copying errors

2. environmental agents:
   - radiation
   - chemicals

Question 18

What are single gene disorders?

Answer 18

Recessive alleles:
in a recessive disorder, you need TWO COPIES of the mutant gene allele for the phenotype to be expressed
e.g phenylketonuria (PKU), Tay-Sachs disease, Albinism

Dominant alleles:
in a dominant disorder, only ONE COPY of the mutant gene allele is necessary for the phenotype to be expressed
e.g polydactyly, huntington’s disease, achondroplasia

Question 19

What is sex-linked inheritance?

Answer 19

• the X and Y chromosomes also carry genes for other phenotype characteristics - not just for sex determination
• the X chromosome, which is much bigger than the Y, carries most of the genes related to other character traits e.g. some blood clotting factors, retinal pigments
• the situation with males is therefore odd: since XY is not homologus any X allele is always expressed in males, whether dominant or recessive
• this is apparent in recessive X-linked disorders where the PHENOTYPE IS ALWAYS EXPRESSED IN THE MALE BUT THE FEMALE IS A CARRIER

Question 20

What are chromosomal mutations?

Answer 20

Nondisjunctions (not coming apart)

meisosis involes TWO cell divisions, meisosis I and II: nondisjunction can occur at either stage

Meiosis I: nondisjunction of homologous chromosomes in meisosis I

Meiosis II: nondisjunction of sister chromatids in meiosis II

Question 21

What are nondisjunctions?

Answer 21

• gametes with abnormal chromosome numbers which produce offspring with abnormal chromosome numbers
  ex. down syndrome

a gamete (i.e a sperm or egg cell) is produced with an extra copy of chromosome 21; the gamete thus has 24 chromosomes

when combined with a normal gamete from the other parent, the embryo now has 47 chromosomes, with three copies of chromosome 21

Question 22

What is the chromosomal disorder of down syndrome? what is that karyotype notation?

Answer 22

47, XX, +21 or 47, XY, +21

Trisomy 21 is the cause of approximately 95% of observed Down syndromes, with 88% coming from nondisjunction in the maternal gamete (egg) and 8% coming from nondisjunction in the paternal (sperm) gamete

Question 23

What is turner syndrome? What is the karyotype notation?

Answer 23

45, XO

female with one X chromosome

• affects 1/5000 births
• lack breast development and other secondary sex characteristics
• sterile
• retarded growth and small body size
• shortened and webbed neck
• does not affect intelligence

Question 24

What is klinefelter syndrome? What is the karyotype notation?

Answer 24

47, XXY

Male with two X chromosomes

• affects 1/500 births
• poorly developed secondary sex characteristics
• breast development
• sterile - small testes and absence of sperm
• taller with abnormally long limbs
• decreased mental ability

Question 25

What is the summary of genetic disorders?

Answer 25

1. phenylketonuria - recessive mutation in phenylalanine metabolizing gene - impaired brain function with mental and physical retardation
2. albinism - recessive mutation in melanin synthesizing genes - lack of pigmentation in skin, eyes and hair
3. tay-sachs disease - recessive mutation in ganglioside metabolizing gene - severe and lethal neurodegeneration
4. huntington’s disease - dominant mutation in a gene of unclear function - neurodegeneration with motor and cognitive impairment
5. down syndrome - 47, XX +21 or 47, XY +21 - mental and physical retardation, sterility
6. turner syndrome - 45, XO - impaired sexual development, sterility
7. klinefelter syndrome, 47, XXY - impaired sexual development, sterility