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Biology 2nd Baccalaureate

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9.1. Genetics basics

Genetics basics

We will begin the topic by recalling some basic concepts of genetics that we already used in past courses.

  • Gen: Mendel called them hereditary factors that controlled a character, such as the color of the seed. According to classical genetics, it is the unit of information that determines a hereditary character. According to molecular genetics, it is a segment of DNA that contains the information necessary for certain amino acids to join in an orderly fashion to form a protein.
  • Locus: It is the place that a gene occupies on the chromosome. In the plural, it is called loci.
  • Alleles: They are each of the different ways in which a gene can be presented at a given locus. For example, one yellow seed allele and one green seed allele.
  • Homozygous or pure race: They are the individuals whose alleles are identical for a locus of homologous chromosomes. These can be both dominant (AA) or recessive (aa).
  • Heterozygous or hybrid race. They are the individuals whose alleles are different (Aa) for a certain character. They are monohybrid if they differ in only one character, dihybrid if they differ in two, or polyhybrid if they differ in several.
  • Genotype: is the set of genes that an organism has, identical in all its cells.
  • Phenotype: It is the external manifestation of the genotype, the characters that are observed. The phenotype can change, as it is influenced by the genotype and the environment.
  • Dominant: Gene that determines the character that is manifested in a heterozygous individual. Dominant alleles are represented by a capital letter.
  • Recessive: A gene that is only expressed in a homozygous individual. The recessive allele does not manifest if the other dominant allele is present. It is represented with the same letter as the dominant one, but in lowercase.
  • Genome: set of genes that are contained in chromosomes. The genome in eukaryotic beings comprises the DNA contained in the nucleus, organized in chromosomes, and the genome of cellular organelles such as mitochondria and chloroplasts; in prokaryotic beings it comprises the DNA of its nucleoid.

Questions that have come out in University entrance exams (Selectividad, EBAU, EvAU)

Aragon. June 2018, option B, question 3.

Regarding Mendelian inheritance: (2.5 points)

a) What is a gene? What is an individual's set of genes called? (0.5 points)

Aragon. September 2017, option B, question 2. (2.5 points)

b) Define the terms: genotype, phenotype, homozygous and heterozygous. (0.5 points)

Aragon. June 2016, option A, question 1.

Short development topic: Mendelian genetics. (3 points)

b) Define the following basic concepts of genetics: (1.5 points; 0.25 points each definition)

Genotype, phenotype.

Locus and allele.

Haploid and diploid.

Homozygous and heterozygous.

Homologous chromosomes.

Aragon. June 2015, option B, question 5.

Define the following concepts: (2 marks)

Allele (0.5 points)

Genotype and phenotype. (0.5 points)

Homozygous and heterozygous. (0.5 points)

Homologous chromosomes. (0.5 points)

Aragon. June 2010, option B. 3.

Explain what you know about the concepts: (2 marks)

b) Allele. (0.5 points)

c) Genotype. (0.5 points)

d) Phenotype. (0.5 points)

Valencian Community, June 2021, question 6.2

Regarding Mendelian genetics (5 points):

a) Define allele.
b) Point out the difference between homozygous and heterozygous organism.
c) Define locus.
d) Point out the difference between dominant inheritance and intermediate inheritance.
e) Indicates the difference between genotype and phenotype.


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