Chapter 9: Genes, chromosomes & patterns of inheritance

Genes, chromosomes & patterns of inheritance

Chapter 9(Unit 4)

Genetic lottery• Genome: total set of genes found in a cell or

individual (study of genome: genomics)• Gametes (germline cells): Egg & Sperm cells• Somatic cells: all the other cells in an organism• Genotype: set of genetic instructions (from parents)• Phenotype: visible expression of the genotype• Gonades: specialised organs which

produce gametes Ovaries & Testes

Chromosomes

Human Chromosomes

• No of chrom. in every cell: 46 DIPLOID• EXCEPT gametes: 23 chrom. HAPLOID• Sex chromosomes (X, Y) XY XX• Other 22 chrom. pairs are called autosomes

and can be distinguished by:– Their relative size– The position of centromere– Patterns of light & dark bands

KaryotypeKaryotype: the ordered arrangement of images of a diploid set of chromosomes

Study of Karyotypes can reveal chromosomal changes, such as:• an additional Chrom.• loss of a Chrom.• duplication or deletion of part of a Chrom.

Homologous chrom.

Changes involving chromosomes• Changes in total number of chromosomes

– Addition of whole chrom. e.g. Down syndrome– Deletion of whole chrom. e.g. Turner syndrome– Deletion of part chrom. e.g. Wolf-Hirschhorn syndrome

• Changes to part of chromosomes– Duplication (part of chrom. duplicated)– Deletion (part of chrom. is missing)

• Re-arrangements of chromosomes– Translocation (part of a chrom. becomes attached to part of

another chrom.)

p. 296

Sex determination• In humans: XX = femal, XY=male• Reptiles: incubation temp. of the eggs• Birds:

Mitosis/Meiosis

Mitosis

Mitosis: source of „growth“• Function: growth and repair• Two genetically identical daughter cells

• Interphase *(G1, S, G2)• Prophase• Metaphase• Anaphase• Telophase• Cytokinesis

*G1 (first gap)

S (synthesis) G2 (second gap)

During all three phases, the cell grows. However, chromosomes are replicated only during the S phase.

M

I

Meiosis

Meiosis: source of variability

• Sexual reproduction produces variability among offspring

• Recombination: re-assortment of genetic material to produce new genetic combinations (crossing over)

Biozone: p. 188 -192 and 213 -216

Apoptosis

Genes – inherited instructions

How many genes?

Number of genes ≠ complexity of organism

Common Name Species Name Number of genes

Human Homo sapiens 20‘000 – 25‘000

Mustard plant Arabidopsis thaliana 27‘000

Nematode worm Caenorhabditis elegans 20‘000

Fruit Fly Drosophila melanogaster 14‘000

Baker‘s yeast Saccharomyces cerevisiae 6000Gut bacterium Escherichia coli 4000

Does genome size matter?Common Name Species Name Approx. Genome size (millions

of base pairs)Human Homo sapiens 3100

Snake Boa constrictor 2100

Onion Allium cepa 18‘000

Fruit Fly Drosophila melanogaster 180

Newt Amphiuma means 84‘000Amoeba Amoeba dubia 670‘000

Lungfish Protopterus aethiopicus 140‘000

Human: out of the 3100 million base pairs, only 2% code for something

Amoeba: keep all genetic information, just in case

Chromosomes in prokaryotes

No non-coding regions!

Mitochondrial DNA (mtDNA)

• Circular DNA molecule (double-stranded)

• Not bound by nuclear envelope

• Not packaged into chromatin

• Hardly any non-coding regions

Location of genes• Locus (loci): Position on chromosome that is

occupied by a gene• Chromosome map: drawing showing gene loci

The region in which a locus is positioned is identified by a system, such as 7q31, where:• The first number (or letter) identifies the chromosome• The letter p or q denoted the chromosome arm (p=short arm, q= long arm)• The final number identifies the region of the chromosome arm

Alleles: particular forms of a gene• A gene that controls one function can exist in

different forms. These different forms are called alleles

• Each different allele is identified by its specific phenotypic action

• Alleles are commonly represented by letters of the alphabet Biozone: p.281

Identifying genotypes

• Homozygous: two identical alleles– Gametes produced will be identical, all having the same allele

• Heterozygous: two different alleles• Gametes produced will be of two kinds, half having one and half

having the other allele

• Sex-linked genotypes:– X-linked (colour vision defects, haemophilia)– Y-linkes (SRY)– X-inactivation

Continuous variation

Discontinuous variation

GREGOR MENDELAustrian monk (1822-84)Father of genetics, who carried out some pioneering work using pea plants

Complete dominance• The dominant allele completely masks the

effect of the recessive allele in heterozygous condition.

• See Human Variation Biozone: p 287Dominant trait Recessive trait

Right-handed (R) Left-handed (r)

Free ear lobes (F) Attached ear lobes (f)

Chin-cleft (C) No chin cleft (c)

Non-red hair (R) Red hair (r)

Incomplete Dominance• Both traits (alleles) are expressed in the

heterozygote• But seen as mixed together

• Eg. White x Red-> Pink

Incomplete Dominance

Co-dominance

• Both traits (alleles) are expressed in the heterozygote

• But seen seperately• Example: Diff. Blood types

Environmental interactionsphenotype = genotype + environment

alkalinepH

Acidic pH

Punnett square• The Punnett square is a diagram that is used

to predict an outcome of a particular cross or breeding experiment

Basic Genetic Crosses• Monohybrid Cross (Ratio: 3:1)• Dihybrid Cross (Ratio: 9:3:3:1)• Test Cross

Monohybrid Cross

• A monohybrid cross involves the alleles at a single gene locus

Ratio for Bb x Bb:3:1

75% violet, 25% white0.75 (3/4) violet0.25 (1/4) white

Dihybrid Cross• A dihybrid cross involves the alleles at two

gene loci

Ratio for RRYY x rryy:9:3:3:1

0.56 (9/16) yellow &round0.18 (3/16) yellow & wrinkled0.18 (3/16) green & round0.06(1/16) green & wrinkles

Test Cross• A way to establish the

genotype of an organism with the dominant phenotype for a particular trait (homozygous or heterozygous?)

• Establish linkage relationships between genes

• Can be either monohybrid or dihybrid

Linked genes• Genes that are close together on a chromosome

(close loci of two genes)

• The two genes will tend to be inherited together, this however can be broken by crossing over (recombination)

• The closer the genes are located on a chromosome, the smaller the chance for recombination.

Pedigree analysis

• A Pedigree analysis can be used to folow the inheritance of traits through a family over a number of generations

Is the condition observed in each generation of a family in which it occurs?

Is the condition mainly in males?

If daughters have the condition does their father also have it?

Autosomal recessive Sex-linked recessive

Do only males have the condition, passing it on from father to son?

Y-linkageDo males with the condition who mate with a normal female have all daughters, but no sons with the condition?

Autosomal dominant Sex-linked dominant

NO

NO

NO

NO

YES

YES

YES

YES