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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