(3.7) Examples of interpreting family trees - family pedigrees

Ex 1. The inheritance of freckles

The tendency to form freckles in sunlight is genetically inherited.

(a) If Molly and Mark have freckles who might you predict to also have freckles in the 3rd generation?

Joey or Selena

(b) If only Phil has freckles in the 1st generation and only Joey in the 3rd generation who is most likely to have freckles in the 2nd generation?

Mark

(c) If only Fred and Sue have freckles in the first two generations, who is most likely to have freckles in the 3rd generation?

Joey or Selena

(d If Fred and Selena have freckles, who most likely to have freckles too?

Sue

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How to use a test cross to identify an unknown genotype

The unknown genotype can be determined by observing the phenotypes of the resulting offspring.

If crossing the unknown dominant phenotype (PP or Pp genotype) individual with the recessive phenotype individual produces only dominant phenotypes (no recessive), then the unknown individual is homozygous dominant.

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2. for future use

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Key points Source of information is based on textbooks & syllabus-specifications for students taking the AQA GCSE, Edexcel GCSE and OCR GCSE level biology examinations (~US grades 9-10).

Key points on how to interpret a family tree of inheritance

Interpreting a Family Tree of Inheritance and Using a Test Cross

Understanding inheritance patterns is crucial for studying genetics.

Family trees (pedigree diagrams) and test crosses help identify the transmission of genetic traits and the genotype of individuals.

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1. How to Interpret a Family Tree of Inheritance

A family tree (pedigree diagram) visually represents the inheritance of traits and genetic disorders over generations. Scientists use these diagrams to study dominant, recessive, and sex-linked traits.

Key Symbols Used in Family Trees:

These are more elaborate symbols than the family tree illustrated above.

Steps to Interpret a Family Tree:

1. Identify Generations: The top row is the oldest generation, and younger generations follow below.

2. Determine Inheritance Pattern:

   • Dominant trait: Appears in every generation.

   • Recessive trait: May skip generations and appear only when both parents carry the allele.

   • Sex-linked trait: If mostly males are affected, it may be X-linked recessive (e.g., hemophilia).

3. Identify Carriers: Unaffected parents producing affected children suggests recessive inheritance.

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2. How to Use a Test Cross to Identify an Unknown Genotype

A test cross helps determine whether an individual with a dominant phenotype has a homozygous dominant (AA) or heterozygous (Aa) genotype.

Steps to Perform a Test Cross:

1. Cross the individual with an unknown genotype (A?) with a homozygous recessive (aa) individual.

2. Analyze offspring outcomes:

   • If all offspring show the dominant trait, the tested individual is likely homozygous dominant (AA).

   • If some offspring show the recessive trait, the tested individual is heterozygous (Aa).

Punnett Square Example – Testing a Dominant Trait (A?):

Genetic Outcomes:

• 100% Dominant Offspring → Individual tested is AA (homozygous dominant).

• 50% Dominant & 50% Recessive Offspring → Individual tested is Aa (heterozygous).

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3. Importance in Human Genetics

Interpreting pedigrees and using test crosses are vital for:

• Identifying Genetic Disorders: Used in medical research to track conditions like cystic fibrosis and sickle cell anemia.

• Genetic Counseling: Helps families assess risks before having children.

• Studying Evolution & Natural Selection: Reveals inheritance patterns in populations.

• Advancing Genetic Research: Supports selective breeding and personalized medicine.

Mastering pedigree analysis and test crosses enhances understanding of inheritance.

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Summary of learning objectives and key words or phrases

In terms of genetic inheritance, be able to interpret a family tree e.g. the inheritance of freckles

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